diff --git a/data/scenarios/Testing/144-subworlds/basic-subworld.yaml b/data/scenarios/Testing/144-subworlds/basic-subworld.yaml
index 18b6e27dd7..aa6c9c4e0c 100644
--- a/data/scenarios/Testing/144-subworlds/basic-subworld.yaml
+++ b/data/scenarios/Testing/144-subworlds/basic-subworld.yaml
@@ -82,6 +82,7 @@ subworlds:
       .p..f...P.
       b..b..b..b
 world:
+  name: root
   default: [blank]
   palette:
     '.': [grass]
diff --git a/data/scenarios/Testing/144-subworlds/subworld-located-robots.yaml b/data/scenarios/Testing/144-subworlds/subworld-located-robots.yaml
index c48d6a3747..2d1b831461 100644
--- a/data/scenarios/Testing/144-subworlds/subworld-located-robots.yaml
+++ b/data/scenarios/Testing/144-subworlds/subworld-located-robots.yaml
@@ -89,6 +89,7 @@ subworlds:
       .p......P.
       b..b..b..b
 world:
+  name: root
   default: [blank]
   palette:
     '.': [grass]
diff --git a/data/scenarios/Testing/144-subworlds/subworld-mapped-robots.yaml b/data/scenarios/Testing/144-subworlds/subworld-mapped-robots.yaml
index 7cce067027..5d26158066 100644
--- a/data/scenarios/Testing/144-subworlds/subworld-mapped-robots.yaml
+++ b/data/scenarios/Testing/144-subworlds/subworld-mapped-robots.yaml
@@ -90,6 +90,7 @@ subworlds:
       .p.t....P.
       b..b..b..b
 world:
+  name: root
   default: [blank]
   palette:
     '.': [grass]
diff --git a/data/scenarios/Testing/144-subworlds/subworld-shared-structures.yaml b/data/scenarios/Testing/144-subworlds/subworld-shared-structures.yaml
index 69e2438e80..5e6759b7b7 100644
--- a/data/scenarios/Testing/144-subworlds/subworld-shared-structures.yaml
+++ b/data/scenarios/Testing/144-subworlds/subworld-shared-structures.yaml
@@ -170,6 +170,7 @@ subworlds:
       .....
       .....
 world:
+  name: root
   default: [blank]
   palette:
     '.': [grass]
diff --git a/data/scenarios/Testing/_Validation/144-subworld-uniqueness-enforcement-default.yaml b/data/scenarios/Testing/_Validation/144-subworld-uniqueness-enforcement-default.yaml
new file mode 100644
index 0000000000..062101a5d6
--- /dev/null
+++ b/data/scenarios/Testing/_Validation/144-subworld-uniqueness-enforcement-default.yaml
@@ -0,0 +1,87 @@
+version: 1
+name: Subworld uniqueness (default name)
+description: |
+  Has two unnamed subworlds, which fail uniquenss
+attrs:
+  - name: portal_in
+    fg: "#ff9a00"
+    bg: "#ff5d00"
+  - name: portal_out
+    fg: "#00a2ff"
+    bg: "#0065ff"
+entities:
+  - name: telepad entrance
+    display:
+      attr: portal_in
+      char: "o"
+    description:
+    - Portal entrance
+    properties: [known]
+  - name: telepad exit
+    display:
+      attr: portal_out
+      char: "o"
+    description:
+    - Portal exit
+    properties: [known]
+robots:
+  - name: base
+    dir: [1, 0]
+    devices:
+      - ADT calculator
+      - branch predictor
+      - comparator
+      - compass
+      - dictionary
+      - GPS receiver
+      - grabber
+      - lambda
+      - lodestone
+      - logger
+      - strange loop
+      - treads
+  - name: judge
+    dir: [1, 0]
+    system: true
+    display:
+      char: 'J'
+      invisible: true
+known: [flower, boulder]
+subworlds:
+  - default: [blank]
+    palette:
+      '.': [dirt]
+      'f': [dirt, flower]
+      'b': [dirt, boulder]
+      'p':
+        cell: [dirt, telepad exit]
+        waypoint:
+          name: portal_out2
+      'P':
+        cell: [dirt, telepad entrance]
+        waypoint:
+          name: portal_in2
+    upperleft: [-1, 1]
+    map: |
+      b..b..b..b
+      .p..f...P.
+      b..b..b..b
+world:
+  default: [blank]
+  palette:
+    '.': [grass]
+    'B': [grass, null, base]
+    't': [ice, null, judge]
+    'p':
+      cell: [grass, telepad exit]
+      waypoint:
+        name: portal_out1
+    'P':
+      cell: [grass, telepad entrance]
+      waypoint:
+        name: portal_in1
+  upperleft: [-1, 1]
+  map: |
+    ..........
+    .p.Bt...P.
+    ..........
diff --git a/data/scenarios/Testing/_Validation/144-subworld-uniqueness-enforcement-named.yaml b/data/scenarios/Testing/_Validation/144-subworld-uniqueness-enforcement-named.yaml
new file mode 100644
index 0000000000..b608da8ec1
--- /dev/null
+++ b/data/scenarios/Testing/_Validation/144-subworld-uniqueness-enforcement-named.yaml
@@ -0,0 +1,89 @@
+version: 1
+name: Subworld uniqueness (explicit name)
+description: |
+  Has two identically-named subworlds
+attrs:
+  - name: portal_in
+    fg: "#ff9a00"
+    bg: "#ff5d00"
+  - name: portal_out
+    fg: "#00a2ff"
+    bg: "#0065ff"
+entities:
+  - name: telepad entrance
+    display:
+      attr: portal_in
+      char: "o"
+    description:
+    - Portal entrance
+    properties: [known]
+  - name: telepad exit
+    display:
+      attr: portal_out
+      char: "o"
+    description:
+    - Portal exit
+    properties: [known]
+robots:
+  - name: base
+    dir: [1, 0]
+    devices:
+      - ADT calculator
+      - branch predictor
+      - comparator
+      - compass
+      - dictionary
+      - GPS receiver
+      - grabber
+      - lambda
+      - lodestone
+      - logger
+      - strange loop
+      - treads
+  - name: judge
+    dir: [1, 0]
+    system: true
+    display:
+      char: 'J'
+      invisible: true
+known: [flower, boulder]
+subworlds:
+  - name: foo
+    default: [blank]
+    palette:
+      '.': [dirt]
+      'f': [dirt, flower]
+      'b': [dirt, boulder]
+      'p':
+        cell: [dirt, telepad exit]
+        waypoint:
+          name: portal_out2
+      'P':
+        cell: [dirt, telepad entrance]
+        waypoint:
+          name: portal_in2
+    upperleft: [-1, 1]
+    map: |
+      b..b..b..b
+      .p..f...P.
+      b..b..b..b
+world:
+  name: foo
+  default: [blank]
+  palette:
+    '.': [grass]
+    'B': [grass, null, base]
+    't': [ice, null, judge]
+    'p':
+      cell: [grass, telepad exit]
+      waypoint:
+        name: portal_out1
+    'P':
+      cell: [grass, telepad entrance]
+      waypoint:
+        name: portal_in1
+  upperleft: [-1, 1]
+  map: |
+    ..........
+    .p.Bt...P.
+    ..........
diff --git a/src/Swarm/Game/Scenario.hs b/src/Swarm/Game/Scenario.hs
index 55e170b8de..a13020f811 100644
--- a/src/Swarm/Game/Scenario.hs
+++ b/src/Swarm/Game/Scenario.hs
@@ -48,7 +48,7 @@ module Swarm.Game.Scenario (
 
 import Control.Arrow ((&&&))
 import Control.Lens hiding (from, (.=), (<.>))
-import Control.Monad (filterM)
+import Control.Monad (filterM, unless)
 import Control.Monad.Except (ExceptT (..), runExceptT, withExceptT)
 import Control.Monad.IO.Class (MonadIO (liftIO))
 import Control.Monad.Trans.Except (except)
@@ -77,7 +77,7 @@ import Swarm.Game.Scenario.Topography.Structure qualified as Structure
 import Swarm.Game.Scenario.Topography.WorldDescription
 import Swarm.Game.Universe
 import Swarm.Language.Pipeline (ProcessedTerm)
-import Swarm.Util (failT)
+import Swarm.Util (binTuples, failT)
 import Swarm.Util.Lens (makeLensesNoSigs)
 import Swarm.Util.Yaml
 import System.Directory (doesFileExist)
@@ -140,6 +140,16 @@ instance FromJSONE EntityMap Scenario where
         subworlds <- v ..:? "subworlds" ..!= []
         return $ rootWorld :| subworlds
 
+      let worldsByName = binTuples $ NE.toList $ NE.map (worldName &&& id) allWorlds
+          dupedNames = M.keys $ M.filter ((> 1) . length) worldsByName
+      unless (null dupedNames)
+        . fail
+        . T.unpack
+        $ T.unwords
+          [ "Subworld names are not unique:"
+          , T.intercalate ", " $ map renderWorldName dupedNames
+          ]
+
       let mergedWaypoints =
             M.fromList $
               map (worldName &&& runIdentity . waypoints . navigation) $
diff --git a/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs b/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs
index 253e12b631..c5f2903f68 100644
--- a/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs
+++ b/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs
@@ -165,7 +165,7 @@ validatePortals (Navigation wpUniverse partialPortals) = do
 
   return $ M.fromList portalPairs
  where
-  getLocs (Cosmo swName@(SubworldName rawSwName) wpWrapper@(WaypointName exitName)) = do
+  getLocs (Cosmo swName wpWrapper@(WaypointName exitName)) = do
     subworldWaypoints <- case M.lookup swName wpUniverse of
       Just x -> return x
       Nothing ->
@@ -175,7 +175,7 @@ validatePortals (Navigation wpUniverse partialPortals) = do
               [ "Could not lookup waypoint"
               , quote exitName
               , "for portal exit because subworld"
-              , quote rawSwName
+              , quote $ renderWorldName swName
               , "does not exist"
               ]
 
diff --git a/src/Swarm/Game/Scenario/Topography/WorldDescription.hs b/src/Swarm/Game/Scenario/Topography/WorldDescription.hs
index 3cb4f7ab0b..a83c3e1d53 100644
--- a/src/Swarm/Game/Scenario/Topography/WorldDescription.hs
+++ b/src/Swarm/Game/Scenario/Topography/WorldDescription.hs
@@ -59,7 +59,7 @@ instance FromJSONE (InheritedStructureDefs, (EntityMap, RobotMap)) WorldDescript
     (initialArea, mapWaypoints) <- liftE ((v .:? "map" .!= "") >>= Structure.paintMap Nothing pal)
 
     upperLeft <- liftE (v .:? "upperleft" .!= origin)
-    subWorldName <- liftE (v .:? "name" .!= defaultRootSubworldName)
+    subWorldName <- liftE (v .:? "name" .!= DefaultRootSubworld)
 
     let initialStructureDefs = scnenarioLevelStructureDefs <> rootWorldStructureDefs
         struc = Structure initialArea initialStructureDefs placementDefs $ waypointDefs <> mapWaypoints
diff --git a/src/Swarm/Game/Universe.hs b/src/Swarm/Game/Universe.hs
index 71b2b4b265..8cdab8a9a9 100644
--- a/src/Swarm/Game/Universe.hs
+++ b/src/Swarm/Game/Universe.hs
@@ -9,28 +9,21 @@ import Control.Lens (makeLenses, view)
 import Data.Function (on)
 import Data.Int (Int32)
 import Data.Text (Text)
-import Data.Yaml (FromJSON, ToJSON, Value (Object), parseJSON, (.:))
+import Data.Yaml (FromJSON, ToJSON, Value (Object), parseJSON, withText, (.:))
 import GHC.Generics (Generic)
 import Linear (V2 (..))
 import Swarm.Game.Location
 
--- TODO: It would be better not to use this as
--- an "in-band" reserved name. Preferably its use
--- should be eliminated entirely.
--- E.g., robot locations specified within a toplevel robot
--- definition (instead of on a map) should require the subworld
--- to be specified.  Currently, the subworld name is optional
--- for backwards compatibility.
--- If, after all, a "default" is still required, should
--- use a dedicated sum type member to designate
--- the "default" for cases in which it is required.
-defaultRootSubworldName :: SubworldName
-defaultRootSubworldName = SubworldName "root"
+data SubworldName = DefaultRootSubworld | SubworldName Text
+  deriving (Show, Eq, Ord, Generic, ToJSON)
 
--- | Note: The primary overworld shall use
--- the reserved name \"root\".
-newtype SubworldName = SubworldName Text
-  deriving (Show, Eq, Ord, Generic, FromJSON, ToJSON)
+instance FromJSON SubworldName where
+  parseJSON = withText "subworld name" $ return . SubworldName
+
+renderWorldName :: SubworldName -> Text
+renderWorldName = \case
+  SubworldName s -> s
+  DefaultRootSubworld -> "<default>"
 
 -- | The swarm universe consists of locations
 -- indexed by subworld.
@@ -47,7 +40,7 @@ makeLenses ''Cosmo
 instance (FromJSON a) => FromJSON (Cosmo a) where
   parseJSON x = case x of
     Object v -> objParse v
-    _ -> Cosmo defaultRootSubworldName <$> parseJSON x
+    _ -> Cosmo DefaultRootSubworld <$> parseJSON x
    where
     objParse v =
       Cosmo
@@ -55,7 +48,7 @@ instance (FromJSON a) => FromJSON (Cosmo a) where
         <*> v .: "loc"
 
 defaultCosmoLocation :: Cosmo Location
-defaultCosmoLocation = Cosmo defaultRootSubworldName origin
+defaultCosmoLocation = Cosmo DefaultRootSubworld origin
 
 data DistanceMeasure b = Measurable b | InfinitelyFar
   deriving (Eq, Ord)
diff --git a/src/Swarm/TUI/Editor/Model.hs b/src/Swarm/TUI/Editor/Model.hs
index 8f62befba9..4a335d8e7c 100644
--- a/src/Swarm/TUI/Editor/Model.hs
+++ b/src/Swarm/TUI/Editor/Model.hs
@@ -83,6 +83,6 @@ initialWorldEditor ts =
     MapEditingBounds
       -- Note that these are in "world coordinates",
       -- not in player-facing "Location" coordinates
-      (Just $ Cosmo defaultRootSubworldName (W.Coords (-10, -20), W.Coords (10, 20)))
+      (Just $ Cosmo DefaultRootSubworld (W.Coords (-10, -20), W.Coords (10, 20)))
       (ts - 1)
       SelectionComplete
diff --git a/src/Swarm/TUI/Editor/Palette.hs b/src/Swarm/TUI/Editor/Palette.hs
index d416a1d4a8..122fe0bbc8 100644
--- a/src/Swarm/TUI/Editor/Palette.hs
+++ b/src/Swarm/TUI/Editor/Palette.hs
@@ -129,7 +129,7 @@ constructScenario maybeOriginalScenario cellGrid =
       , ul = upperLeftCoord
       , area = cellGrid
       , navigation = Navigation mempty mempty
-      , worldName = defaultRootSubworldName
+      , worldName = DefaultRootSubworld
       }
 
   suggestedPalette = makeSuggestedPalette maybeOriginalScenario cellGrid
diff --git a/src/Swarm/TUI/Editor/Util.hs b/src/Swarm/TUI/Editor/Util.hs
index 1e832ff36a..b9eb365b7c 100644
--- a/src/Swarm/TUI/Editor/Util.hs
+++ b/src/Swarm/TUI/Editor/Util.hs
@@ -29,7 +29,7 @@ getEditingBounds :: WorldDescription -> (Bool, Cosmo W.BoundsRectangle)
 getEditingBounds myWorld =
   (EA.isEmpty a, newBounds)
  where
-  newBounds = Cosmo defaultRootSubworldName (W.locToCoords upperLeftLoc, W.locToCoords lowerRightLoc)
+  newBounds = Cosmo DefaultRootSubworld (W.locToCoords upperLeftLoc, W.locToCoords lowerRightLoc)
   upperLeftLoc = ul myWorld
   a = EA.getAreaDimensions $ area myWorld
   lowerRightLoc = EA.upperLeftToBottomRight a upperLeftLoc
diff --git a/src/Swarm/TUI/View.hs b/src/Swarm/TUI/View.hs
index 9a566a0a37..bb9d86b673 100644
--- a/src/Swarm/TUI/View.hs
+++ b/src/Swarm/TUI/View.hs
@@ -467,10 +467,10 @@ renderCoordsString cCoords =
   unwords
     [ VU.locationToString coords
     , "in"
-    , T.unpack swName
+    , T.unpack $ renderWorldName sw
     ]
  where
-  Cosmo (SubworldName swName) coords = cCoords
+  Cosmo sw coords = cCoords
 
 drawWorldCursorInfo :: WorldEditor Name -> GameState -> Cosmo W.Coords -> Widget Name
 drawWorldCursorInfo worldEditor g cCoords =