diff --git a/src/Domain/Battlefield.php b/src/Domain/Battlefield.php
index 5f70143..c22eb2e 100644
--- a/src/Domain/Battlefield.php
+++ b/src/Domain/Battlefield.php
@@ -13,7 +13,7 @@ final readonly class Battlefield
     private array $terrain;
 
     /**
-     * @param array<array-key, mixed> $terrain
+     * @param array<string, Terrain> $terrain
      */
     public function __construct(
         public int $width,
@@ -27,19 +27,11 @@ final readonly class Battlefield
         $validatedTerrain = [];
 
         foreach ($terrain as $key => $terrainType) {
-            if (!$terrainType instanceof Terrain) {
+            if (!self::isTerrain($terrainType)) {
                 throw new InvalidArgumentException("Invalid terrain value at coordinate {$key}.");
             }
 
-            if (!is_string($key) || preg_match('/^(\d+):(\d+)$/', $key, $matches) !== 1) {
-                throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
-            }
-
-            $position = new Position((int) $matches[1], (int) $matches[2]);
-
-            if ($key !== $position->key()) {
-                throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
-            }
+            $position = self::positionFromTerrainKey($key);
 
             if (!$this->contains($position)) {
                 throw new InvalidArgumentException("Terrain coordinate {$key} is outside the battlefield.");
@@ -51,6 +43,26 @@ final readonly class Battlefield
         $this->terrain = $validatedTerrain;
     }
 
+    private static function isTerrain(mixed $terrain): bool
+    {
+        return $terrain instanceof Terrain;
+    }
+
+    private static function positionFromTerrainKey(mixed $key): Position
+    {
+        if (!is_string($key) || preg_match('/^(\d+):(\d+)$/', $key, $matches) !== 1) {
+            throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
+        }
+
+        $position = new Position((int) $matches[1], (int) $matches[2]);
+
+        if ($key !== $position->key()) {
+            throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
+        }
+
+        return $position;
+    }
+
     public function contains(Position $position): bool
     {
         return $position->x >= 0
diff --git a/tests/Unit/Domain/BattlefieldTest.php b/tests/Unit/Domain/BattlefieldTest.php
index f6061d0..d9089f4 100644
--- a/tests/Unit/Domain/BattlefieldTest.php
+++ b/tests/Unit/Domain/BattlefieldTest.php
@@ -19,6 +19,22 @@ final class BattlefieldTest extends TestCase
         new Battlefield(7, 16);
     }
 
+    public function testItAcceptsMinimumAndMaximumDimensions(): void
+    {
+        $minimum = new Battlefield(8, 8);
+        $maximum = new Battlefield(16, 16);
+
+        self::assertSame([8, 8], [$minimum->width, $minimum->height]);
+        self::assertSame([16, 16], [$maximum->width, $maximum->height]);
+    }
+
+    public function testItRejectsDimensionsAboveTheSupportedRange(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        new Battlefield(16, 17);
+    }
+
     public function testItRejectsNonCanonicalTerrainCoordinates(): void
     {
         $this->expectException(InvalidArgumentException::class);
@@ -37,9 +53,8 @@ final class BattlefieldTest extends TestCase
     {
         $this->expectException(InvalidArgumentException::class);
 
-        $terrain = ['0:0' => 'forest'];
-
-        new Battlefield(8, 8, $terrain);
+        // @phpstan-ignore argument.type
+        new Battlefield(8, 8, ['0:0' => 'forest']);
     }
 
     public function testReachableRejectsAStartOutsideTheBattlefield(): void
@@ -90,4 +105,17 @@ final class BattlefieldTest extends TestCase
             '0:1' => 2,
         ], $reachable);
     }
+
+    public function testReachableReplacesAHighCostRouteWithALowerCostDetour(): void
+    {
+        $battlefield = new Battlefield(8, 8, [
+            '1:0' => Terrain::Forest,
+            '2:0' => Terrain::Forest,
+            '3:0' => Terrain::Forest,
+        ]);
+
+        $reachable = $battlefield->reachable(new Position(0, 0), 7, []);
+
+        self::assertSame(6, $reachable['4:0']);
+    }
 }