diff --git a/dlgr/griduniverse/csv_gridworlds.py b/dlgr/griduniverse/csv_gridworlds.py
new file mode 100644
index 00000000..b5cf66bf
--- /dev/null
+++ b/dlgr/griduniverse/csv_gridworlds.py
@@ -0,0 +1,84 @@
+import re
+from collections import defaultdict
+
+from dlgr.griduniverse.experiment import Gridworld
+
+player_regex = re.compile(r"(p\d+)(c\d+)?")
+color_names = Gridworld.player_color_names
+
+
+def matrix2gridworld(matrix):
+    """Transform a 2D matrix representing an initial grid state
+    into the serialized format used by Gridworld.
+
+    Example:
+
+    +---------------+---------+--------------------+
+    | w             | stone   | gooseberry_bush|3  |
+    | p1c1          | w       | w                  |
+    |               |         | p3c2               |
+    |               | p4c2    |                    |
+    | big_hard_rock | w       | p2c1               |
+    +---------------+---------+--------------------+
+
+    Explanation:
+
+    - "w": a wall
+    - "stone": item defined by item_id "stone" in game_config.yml
+    - "gooseberry_bush|3": similar to the above, with the added detail
+        that the item has 3 remaining uses
+    - "p2c1": player ID 2, who is on team (color) 1
+    - Empty cells: empty space in the grid
+    """
+    result = defaultdict(list)
+
+    result["rows"] = len(matrix)
+    if matrix:
+        result["columns"] = len(matrix[0])
+    else:
+        result["columns"] = 0
+
+    for row_num, row in enumerate(matrix):
+        for col_num, cell in enumerate(row):
+            # NB: we use [y, x] format in GU!! (╯°□°)╯︵ ┻━┻
+            position = [row_num, col_num]
+            cell = cell.strip()
+            player_match = player_regex.match(cell)
+            if not cell:
+                # emtpy
+                continue
+            if cell == "w":
+                result["walls"].append(position)
+            elif player_match:
+                id_str, color_str = player_match.groups()
+                player_id = id_str.replace("p", "")
+                player_data = {
+                    "id": player_id,
+                    "position": position,
+                }
+                if color_str is not None:
+                    player_color_index = int(color_str.replace("c", "")) - 1
+                    try:
+                        player_data["color"] = color_names[player_color_index]
+                    except IndexError:
+                        max_color = len(color_names)
+                        raise ValueError(
+                            f'Invalid player color specified in "{cell}" at postion {position}. '
+                            f"Max color value is {max_color}, "
+                            f"but you specified {player_color_index + 1}."
+                        )
+
+                result["players"].append(player_data)
+            else:
+                # assume an Item
+                id_and_maybe_uses = [s.strip() for s in cell.split("|")]
+                item_data = {
+                    "id": len(result["items"]) + 1,
+                    "item_id": id_and_maybe_uses[0],
+                    "position": position,
+                }
+                if len(id_and_maybe_uses) == 2:
+                    item_data["remaining_uses"] = int(id_and_maybe_uses[1])
+                result["items"].append(item_data)
+
+    return dict(result)
diff --git a/dlgr/griduniverse/experiment.py b/dlgr/griduniverse/experiment.py
index 9792a113..4d649144 100644
--- a/dlgr/griduniverse/experiment.py
+++ b/dlgr/griduniverse/experiment.py
@@ -1,6 +1,7 @@
 """The Griduniverse."""
 
 import collections
+import csv
 import datetime
 import itertools
 import json
@@ -94,6 +95,7 @@
     "difi_group_label": unicode,
     "difi_group_image": unicode,
     "fun_survey": bool,
+    "map_csv": unicode,
     "pre_difi_question": bool,
     "pre_difi_group_label": unicode,
     "pre_difi_group_image": unicode,
@@ -507,6 +509,18 @@ def compute_payoffs(self):
             player.payoff *= inter_proportions[player.color_idx]
             player.payoff *= self.dollars_per_point
 
+    def load_map(self, csv_file_path):
+        with open(csv_file_path) as csv_file:
+            grid_state = self.csv_to_grid_state(csv_file)
+        self.deserialize(grid_state)
+
+    def csv_to_grid_state(self, csv_file):
+        from .csv_gridworlds import matrix2gridworld  # avoid circular import
+
+        reader = csv.reader(csv_file)
+        grid_state = matrix2gridworld(list(reader))
+        return grid_state
+
     def build_labyrinth(self):
         if self.walls_density and not self.wall_locations:
             start = time.time()
@@ -561,7 +575,7 @@ def deserialize(self, state):
                     self.columns,
                 )
             )
-        self.round = state["round"]
+        self.round = state.get("round", 0)
         # @@@ can't set donation_active because it's a property
         # self.donation_active = state['donation_active']
 
@@ -857,7 +871,7 @@ class Item:
     """
 
     item_config: dict
-    id: int = field(default_factory=lambda: uuid.uuid4())
+    id: int = field(default_factory=lambda: uuid.uuid4().int)
     creation_timestamp: float = field(default_factory=time.time)
     position: tuple = (0, 0)
     remaining_uses: int = field(default=None)
@@ -1353,7 +1367,7 @@ def handle_connect(self, msg):
             return
 
         logger.info("Client {} has connected.".format(player_id))
-        client_count = len(self.grid.players)
+        client_count = len(self.node_by_player_id)
         logger.info("Grid num players: {}".format(self.grid.num_players))
         if client_count < self.grid.num_players:
             participant = self.session.query(dallinger.models.Participant).get(
@@ -1370,13 +1384,14 @@ def handle_connect(self, msg):
                 # We use the current node id modulo the number of colours
                 # to pick the user's colour. This ensures that players are
                 # allocated to colours uniformly.
-                self.grid.spawn_player(
-                    id=player_id,
-                    color_name=self.grid.limited_player_color_names[
-                        node.id % self.grid.num_colors
-                    ],
-                    recruiter_id=participant.recruiter_id,
-                )
+                if player_id not in self.grid.players:
+                    self.grid.spawn_player(
+                        id=player_id,
+                        color_name=self.grid.limited_player_color_names[
+                            node.id % self.grid.num_colors
+                        ],
+                        recruiter_id=participant.recruiter_id,
+                    )
             else:
                 logger.info("No free network found for player {}".format(player_id))
 
@@ -1721,7 +1736,10 @@ def send_state_thread(self):
     def game_loop(self):
         """Update the world state."""
         gevent.sleep(0.1)
-        if not self.config.get("replay", False):
+        map_csv_path = self.config.get("map_csv", None)
+        if map_csv_path is not None:
+            self.grid.load_map(map_csv_path)
+        elif not self.config.get("replay", False):
             self.grid.build_labyrinth()
             logger.info("Spawning items")
             for item_type in self.item_config.values():
diff --git a/package.json b/package.json
index 935e8859..ac54df54 100644
--- a/package.json
+++ b/package.json
@@ -50,5 +50,6 @@
   "bugs": {
     "url": "https://github.com/Dallinger/Griduniverse/issues"
   },
-  "homepage": "https://github.com/Dallinger/Griduniverse#readme"
+  "homepage": "https://github.com/Dallinger/Griduniverse#readme",
+  "packageManager": "yarn@1.22.22+sha512.a6b2f7906b721bba3d67d4aff083df04dad64c399707841b7acf00f6b133b7ac24255f2652fa22ae3534329dc6180534e98d17432037ff6fd140556e2bb3137e"
 }
diff --git a/test/conftest.py b/test/conftest.py
index bcc17403..dc17169b 100644
--- a/test/conftest.py
+++ b/test/conftest.py
@@ -85,9 +85,14 @@ def stub_config():
     }
     from dallinger.config import Configuration, default_keys
 
+    from dlgr.griduniverse.experiment import GU_PARAMS
+
     config = Configuration()
     for key in default_keys:
         config.register(*key)
+    for key in GU_PARAMS.items():
+        config.register(*key)
+
     config.extend(defaults.copy())
     # Patch load() so we don't update any key/value pairs from actual files:
     # (Note: this is blindly cargo-culted in from dallinger's equivalent fixture.
diff --git a/test/test_griduniverse.py b/test/test_griduniverse.py
index 9eb3559d..6af39c79 100755
--- a/test/test_griduniverse.py
+++ b/test/test_griduniverse.py
@@ -2,9 +2,9 @@
 Tests for `dlgr.griduniverse` module.
 """
 import collections
+import csv
 import json
 import time
-import uuid
 
 import mock
 import pytest
@@ -48,7 +48,7 @@ def test_initialized_with_some_default_values(self, item_config):
         item = self.subject(item_config)
 
         assert isinstance(item.creation_timestamp, float)
-        assert isinstance(item.id, uuid.UUID)
+        assert isinstance(item.id, int)
         assert item.position == (0, 0)
 
     def test_instance_specific_values_can_be_specified(self, item_config):
@@ -220,6 +220,65 @@ def test_loop_spawns_items(self, loop_exp_3x):
             [i["item_count"] for i in exp.item_config.values()]
         )
 
+    def test_builds_grid_from_csv_if_specified(self, tmpdir, loop_exp_3x):
+        exp = loop_exp_3x
+        grid_config = [["w", "stone", "", "gooseberry_bush|3", "p1c2"]]
+        # Grid size must match incoming data, so update the gridworlds's existing
+        # settings:
+        exp.grid.rows = len(grid_config)
+        exp.grid.columns = len(grid_config[0])
+
+        csv_file = tmpdir.join("test_grid.csv")
+
+        with csv_file.open(mode="w") as file:
+            writer = csv.writer(file)
+            writer.writerows(grid_config)
+
+        # active_config.extend({"map_csv": csv_file.strpath}, strict=True)
+        exp.config.extend({"map_csv": csv_file.strpath}, strict=True)
+
+        exp.game_loop()
+
+        state = exp.grid.serialize()
+
+        def relevant_keys(dictionary):
+            relevant = {"id", "item_id", "position", "remaining_uses", "color"}
+            return {k: v for k, v in dictionary.items() if k in relevant}
+
+        # Ignore keys added by experiment execution we don't care about and/or
+        # which are non-deterministic (like player names):
+        state["items"] = [relevant_keys(item) for item in state["items"]]
+        state["players"] = [relevant_keys(player) for player in state["players"]]
+
+        assert state == {
+            "columns": 5,
+            "donation_active": False,
+            "items": [
+                {
+                    "id": 1,
+                    "item_id": "stone",
+                    "position": [0, 1],
+                    "remaining_uses": 1,
+                },
+                {
+                    "id": 2,
+                    "item_id": "gooseberry_bush",
+                    "position": [0, 3],
+                    "remaining_uses": 3,
+                },
+            ],
+            "players": [
+                {
+                    "color": "YELLOW",
+                    "id": "1",
+                    "position": [0, 4],
+                }
+            ],
+            "round": 0,
+            "rows": 1,
+            "walls": [[0, 0]],
+        }
+
     def test_loop_serialized_and_saves(self, loop_exp_3x):
         # Grid serialized and added to DB session once per loop
         exp = loop_exp_3x
@@ -285,6 +344,16 @@ def test_handle_connect_adds_player_to_grid(self, exp, a):
         exp.handle_connect({"player_id": participant.id})
         assert participant.id in exp.grid.players
 
+    def test_handle_connect_uses_existing_player_on_grid(self, exp, a):
+        participant = a.participant()
+        exp.grid.players[participant.id] = Player(
+            id=participant.id, color=[0.50, 0.86, 1.00], location=[10, 10]
+        )
+        exp.handle_connect({"player_id": participant.id})
+        assert participant.id in exp.node_by_player_id
+        assert len(exp.grid.players) == 1
+        assert len(exp.node_by_player_id) == 1
+
     def test_handle_connect_is_noop_for_spectators(self, exp):
         exp.handle_connect({"player_id": "spectator"})
         assert exp.node_by_player_id == {}
diff --git a/test/test_gridworld.py b/test/test_gridworld.py
index 3630acd0..093d9dc0 100644
--- a/test/test_gridworld.py
+++ b/test/test_gridworld.py
@@ -157,3 +157,148 @@ def test_instructions(self, gridworld):
         # Just test something basic
         html = gridworld.instructions()
         assert "🫐 Gooseberry (3 points)" in html
+
+
+class TestMatrix2SerializedGridworld(object):
+    """Tests for converting a list of lists extracted from matrix
+    representation of initial grid state into the format used in
+    Gridworld [de]serialization.
+    """
+
+    @property
+    def subject(self):
+        from dlgr.griduniverse.csv_gridworlds import matrix2gridworld
+
+        return matrix2gridworld
+
+    def test_one_row(self):
+        csv = [["w", "stone", "", "gooseberry_bush|3", "p1c2"]]
+
+        result = self.subject(csv)
+
+        assert result == {
+            "columns": 5,
+            "items": [
+                {"id": 1, "item_id": "stone", "position": [0, 1]},
+                {
+                    "id": 2,
+                    "item_id": "gooseberry_bush",
+                    "position": [0, 3],
+                    "remaining_uses": 3,
+                },
+            ],
+            "players": [{"color": "YELLOW", "id": "1", "position": [0, 4]}],
+            "rows": 1,
+            "walls": [[0, 0]],
+        }
+
+    def test_to_demonstrate_orientation(self):
+        csv = [
+            ["top-left", "top-right"],
+            ["bottom-left", "bottom-right"],
+        ]
+
+        result = self.subject(csv)
+
+        assert result["items"] == [
+            {"id": 1, "item_id": "top-left", "position": [0, 0]},
+            {"id": 2, "item_id": "top-right", "position": [0, 1]},
+            {"id": 3, "item_id": "bottom-left", "position": [1, 0]},
+            {"id": 4, "item_id": "bottom-right", "position": [1, 1]},
+        ]
+
+    def test_multirow(self):
+        csv = [
+            ["w", "stone", "", "gooseberry_bush|3", "p1c1"],
+            ["", "w", "", "", ""],
+            ["", "p2c1", "w", "", ""],
+            ["", "", "", "w", ""],
+            ["", "", "", "p3c2", "w"],
+            ["", "", "", "", "w"],
+            ["gooseberry_bush|4", "", "", "", ""],
+            ["", "big_hard_rock", "", "", ""],
+            ["", "p4c2", "", "", ""],
+            ["", "", "p5c3", "", ""],
+        ]
+
+        result = self.subject(csv)
+
+        assert result == {
+            "columns": 5,
+            "items": [
+                {"id": 1, "item_id": "stone", "position": [0, 1]},
+                {
+                    "id": 2,
+                    "item_id": "gooseberry_bush",
+                    "position": [0, 3],
+                    "remaining_uses": 3,
+                },
+                {
+                    "id": 3,
+                    "item_id": "gooseberry_bush",
+                    "position": [6, 0],
+                    "remaining_uses": 4,
+                },
+                {"id": 4, "item_id": "big_hard_rock", "position": [7, 1]},
+            ],
+            "players": [
+                {"color": "BLUE", "id": "1", "position": [0, 4]},
+                {"color": "BLUE", "id": "2", "position": [2, 1]},
+                {"color": "YELLOW", "id": "3", "position": [4, 3]},
+                {"color": "YELLOW", "id": "4", "position": [8, 1]},
+                {"color": "ORANGE", "id": "5", "position": [9, 2]},
+            ],
+            "rows": 10,
+            "walls": [[0, 0], [1, 1], [2, 2], [3, 3], [4, 4], [5, 4]],
+        }
+
+    def test_supports_empty_matrix(self):
+        csv = []
+
+        result = self.subject(csv)
+
+        assert result == {"rows": 0, "columns": 0}
+
+    def test_not_confused_by_whitepace(self):
+        csv = [["w ", "  stone", "  ", " gooseberry_bush | 3 ", " p1c2"]]
+
+        result = self.subject(csv)
+
+        assert result == {
+            "columns": 5,
+            "items": [
+                {"id": 1, "item_id": "stone", "position": [0, 1]},
+                {
+                    "id": 2,
+                    "item_id": "gooseberry_bush",
+                    "position": [0, 3],
+                    "remaining_uses": 3,
+                },
+            ],
+            "players": [{"color": "YELLOW", "id": "1", "position": [0, 4]}],
+            "rows": 1,
+            "walls": [[0, 0]],
+        }
+
+    def test_explains_invalid_player_colors(self):
+        csv = [["p1c999"]]
+
+        with pytest.raises(ValueError) as exc_info:
+            self.subject(csv)
+
+        assert exc_info.match("Invalid player color")
+
+    def test_preserves_empty_edge_rows_and_columns(self):
+        csv = [
+            ["", "", "", "", ""],
+            ["", "", "stone", "", ""],
+            ["", "", "", "", ""],
+        ]
+
+        result = self.subject(csv)
+
+        assert result == {
+            "columns": 5,
+            "items": [{"id": 1, "item_id": "stone", "position": [1, 2]}],
+            "rows": 3,
+        }