diff --git a/src/Domain/CombatEngine.php b/src/Domain/CombatEngine.php
index eab9624..b8e65d2 100644
--- a/src/Domain/CombatEngine.php
+++ b/src/Domain/CombatEngine.php
@@ -53,8 +53,7 @@ final class CombatEngine
             throw new CombatException('Target must be an active enemy unit.');
         }
 
-        $distance = abs($attacker->position->x - $target->position->x)
-            + abs($attacker->position->y - $target->position->y);
+        $distance = $attacker->position->distanceTo($target->position);
 
         if ($distance !== 1) {
             throw new CombatException('Target is outside attack range.');
diff --git a/tests/Unit/Domain/CombatEngineTest.php b/tests/Unit/Domain/CombatEngineTest.php
index 773609b..75b11ba 100644
--- a/tests/Unit/Domain/CombatEngineTest.php
+++ b/tests/Unit/Domain/CombatEngineTest.php
@@ -418,6 +418,25 @@ final class CombatEngineTest extends TestCase
         yield 'defeated enemy' => ['bravo', 0];
     }
 
+    public function testAttackDealsCalculatedOpenTerrainDamage(): void
+    {
+        $target = $this->unit('bravo-1', 'bravo', new Position(1, 0), defense: 3);
+        $match = new MatchState(
+            new Battlefield(8, 8),
+            [
+                $this->unit('alpha-1', 'alpha', new Position(0, 0), attack: 8),
+                $target,
+            ],
+            'alpha',
+        );
+
+        $next = (new CombatEngine())->attack($match, 'alpha-1', 'bravo-1');
+
+        self::assertSame(10, $target->health);
+        self::assertSame(5, $next->unit('bravo-1')->health);
+        self::assertSame(['alpha-1 attacked bravo-1 for 5 damage'], $next->actionLog);
+    }
+
     public function testAttackAlwaysDealsAtLeastOneDamage(): void
     {
         $match = new MatchState(