diff --git a/src/Domain/CombatEngine.php b/src/Domain/CombatEngine.php
index b512e0b..f54372d 100644
--- a/src/Domain/CombatEngine.php
+++ b/src/Domain/CombatEngine.php
@@ -4,13 +4,19 @@ declare(strict_types=1);
 
 namespace BattleForge\Domain;
 
+use InvalidArgumentException;
+
 final class CombatEngine
 {
     public function move(MatchState $match, string $unitId, Position $destination): MatchState
     {
         $this->assertMatchActive($match);
 
-        $unit = $match->unit($unitId);
+        try {
+            $unit = $match->unit($unitId);
+        } catch (InvalidArgumentException $exception) {
+            throw new CombatException("Unknown unit: {$unitId}.", previous: $exception);
+        }
         $this->assertCanAct($match, $unit);
 
         $occupied = [];
diff --git a/src/Domain/MatchState.php b/src/Domain/MatchState.php
index 48ed30b..5e2e6f8 100644
--- a/src/Domain/MatchState.php
+++ b/src/Domain/MatchState.php
@@ -30,6 +30,7 @@ final readonly class MatchState
 
         $unitIds = [];
         $teamIds = [];
+        $occupiedPositions = [];
 
         foreach ($units as $unit) {
             if (!self::isUnitState($unit)) {
@@ -40,6 +41,24 @@ final readonly class MatchState
                 throw new InvalidArgumentException("Duplicate unit id: {$unit->id}.");
             }
 
+            if (!$battlefield->contains($unit->position)) {
+                throw new InvalidArgumentException("Unit {$unit->id} is outside the battlefield.");
+            }
+
+            if (!$unit->isDefeated() && $battlefield->terrainAt($unit->position)->movementCost() === null) {
+                throw new InvalidArgumentException("Living unit {$unit->id} must stand on passable terrain.");
+            }
+
+            $positionKey = $unit->position->key();
+
+            if (!$unit->isDefeated() && isset($occupiedPositions[$positionKey])) {
+                throw new InvalidArgumentException("Living units cannot share position {$positionKey}.");
+            }
+
+            if (!$unit->isDefeated()) {
+                $occupiedPositions[$positionKey] = true;
+            }
+
             $unitIds[$unit->id] = true;
             $teamIds[$unit->teamId] = true;
         }
diff --git a/tests/Unit/Domain/CombatEngineTest.php b/tests/Unit/Domain/CombatEngineTest.php
index 0c8542d..e7d433c 100644
--- a/tests/Unit/Domain/CombatEngineTest.php
+++ b/tests/Unit/Domain/CombatEngineTest.php
@@ -9,7 +9,10 @@ use BattleForge\Domain\CombatEngine;
 use BattleForge\Domain\CombatException;
 use BattleForge\Domain\MatchState;
 use BattleForge\Domain\Position;
+use BattleForge\Domain\Terrain;
 use BattleForge\Domain\UnitState;
+use InvalidArgumentException;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
 final class CombatEngineTest extends TestCase
@@ -36,14 +39,149 @@ final class CombatEngineTest extends TestCase
                 $this->unit('bravo-1', 'bravo', new Position(7, 7)),
             ],
             'alpha',
+            actionLog: ['match started'],
         );
 
         $next = (new CombatEngine())->move($match, 'alpha-1', new Position(2, 0));
 
         self::assertSame('0:0', $match->unit('alpha-1')->position->key());
+        self::assertSame(2, $match->unit('alpha-1')->actionsRemaining);
+        self::assertSame(['match started'], $match->actionLog);
         self::assertSame('2:0', $next->unit('alpha-1')->position->key());
         self::assertSame(1, $next->unit('alpha-1')->actionsRemaining);
-        self::assertSame(['alpha-1 moved to 2:0'], $next->actionLog);
+        self::assertSame(['match started', 'alpha-1 moved to 2:0'], $next->actionLog);
+    }
+
+    public function testMoveTranslatesUnknownUnitAtCommandBoundary(): void
+    {
+        $match = $this->match();
+
+        try {
+            (new CombatEngine())->move($match, 'missing', new Position(1, 0));
+            self::fail('Expected an unknown unit exception.');
+        } catch (CombatException $exception) {
+            self::assertSame('Unknown unit: missing.', $exception->getMessage());
+            self::assertInstanceOf(InvalidArgumentException::class, $exception->getPrevious());
+            self::assertSame('Unknown unit id: missing.', $exception->getPrevious()->getMessage());
+        }
+    }
+
+    public function testMoveRejectsCompletedMatchBeforeResolvingUnit(): void
+    {
+        $match = $this->match()->withWinner('alpha');
+
+        $this->expectException(CombatException::class);
+        $this->expectExceptionMessage('The match is already complete.');
+
+        (new CombatEngine())->move($match, 'missing', new Position(1, 0));
+    }
+
+    public function testMoveRejectsDefeatedActor(): void
+    {
+        $match = new MatchState(
+            new Battlefield(8, 8),
+            [
+                $this->unit('alpha-1', 'alpha', new Position(0, 0), health: 0),
+                $this->unit('bravo-1', 'bravo', new Position(7, 7)),
+            ],
+            'alpha',
+        );
+
+        $this->expectException(CombatException::class);
+        $this->expectExceptionMessage('Defeated units cannot act.');
+
+        (new CombatEngine())->move($match, 'alpha-1', new Position(1, 0));
+    }
+
+    public function testMoveRejectsExhaustedActor(): void
+    {
+        $match = new MatchState(
+            new Battlefield(8, 8),
+            [
+                $this->unit('alpha-1', 'alpha', new Position(0, 0), actionsRemaining: 0),
+                $this->unit('bravo-1', 'bravo', new Position(7, 7)),
+            ],
+            'alpha',
+        );
+
+        $this->expectException(CombatException::class);
+        $this->expectExceptionMessage('Unit has no actions remaining.');
+
+        (new CombatEngine())->move($match, 'alpha-1', new Position(1, 0));
+    }
+
+    public function testMoveRejectsCurrentPosition(): void
+    {
+        $match = $this->match();
+
+        $this->expectException(CombatException::class);
+        $this->expectExceptionMessage('Destination is not reachable.');
+
+        (new CombatEngine())->move($match, 'alpha-1', new Position(0, 0));
+    }
+
+    /**
+     * @param array<string, Terrain> $terrain
+     */
+    #[DataProvider('unreachableDestinationProvider')]
+    public function testMoveRejectsUnreachableDestination(array $terrain, Position $destination): void
+    {
+        $match = new MatchState(
+            new Battlefield(8, 8, $terrain),
+            [
+                $this->unit('alpha-1', 'alpha', new Position(0, 0)),
+                $this->unit('bravo-1', 'bravo', new Position(7, 7)),
+            ],
+            'alpha',
+        );
+
+        $this->expectException(CombatException::class);
+        $this->expectExceptionMessage('Destination is not reachable.');
+
+        (new CombatEngine())->move($match, 'alpha-1', $destination);
+    }
+
+    /**
+     * @return iterable<string, array{array<string, Terrain>, Position}>
+     */
+    public static function unreachableDestinationProvider(): iterable
+    {
+        yield 'outside battlefield' => [[], new Position(8, 0)];
+        yield 'outside speed budget' => [[], new Position(5, 0)];
+        yield 'impassable terrain' => [['1:0' => Terrain::Blocking], new Position(1, 0)];
+    }
+
+    public function testDefeatedUnitDoesNotBlockItsPosition(): void
+    {
+        $match = new MatchState(
+            new Battlefield(8, 8),
+            [
+                $this->unit('alpha-1', 'alpha', new Position(0, 0)),
+                $this->unit('alpha-2', 'alpha', new Position(1, 0), health: 0),
+                $this->unit('bravo-1', 'bravo', new Position(7, 7)),
+            ],
+            'alpha',
+        );
+
+        $next = (new CombatEngine())->move($match, 'alpha-1', new Position(1, 0));
+
+        self::assertSame('1:0', $next->unit('alpha-1')->position->key());
+    }
+
+    public function testMoveHonorsTerrainCostWithinSpeedBudget(): void
+    {
+        $match = new MatchState(
+            new Battlefield(8, 8, ['1:0' => Terrain::Forest]),
+            [
+                $this->unit('alpha-1', 'alpha', new Position(0, 0), speed: 3),
+                $this->unit('bravo-1', 'bravo', new Position(7, 7)),
+            ],
+            'alpha',
+        );
+
+        $next = (new CombatEngine())->move($match, 'alpha-1', new Position(2, 0));
+
+        self::assertSame('2:0', $next->unit('alpha-1')->position->key());
     }
 
     public function testMoveRejectsUnitFromInactiveTeam(): void
@@ -81,8 +219,26 @@ final class CombatEngineTest extends TestCase
         (new CombatEngine())->move($match, 'alpha-1', new Position(1, 0));
     }
 
-    private function unit(string $id, string $team, Position $position): UnitState
+    private function match(): MatchState
     {
-        return new UnitState($id, $team, $position, 10, 10, 4, 2, 4, 2);
+        return new MatchState(
+            new Battlefield(8, 8),
+            [
+                $this->unit('alpha-1', 'alpha', new Position(0, 0)),
+                $this->unit('bravo-1', 'bravo', new Position(7, 7)),
+            ],
+            'alpha',
+        );
+    }
+
+    private function unit(
+        string $id,
+        string $team,
+        Position $position,
+        int $health = 10,
+        int $speed = 4,
+        int $actionsRemaining = 2,
+    ): UnitState {
+        return new UnitState($id, $team, $position, 10, $health, 4, 2, $speed, $actionsRemaining);
     }
 }
diff --git a/tests/Unit/Domain/MatchStateTest.php b/tests/Unit/Domain/MatchStateTest.php
index 27eb949..f22f634 100644
--- a/tests/Unit/Domain/MatchStateTest.php
+++ b/tests/Unit/Domain/MatchStateTest.php
@@ -7,8 +7,10 @@ namespace BattleForge\Tests\Unit\Domain;
 use BattleForge\Domain\Battlefield;
 use BattleForge\Domain\MatchState;
 use BattleForge\Domain\Position;
+use BattleForge\Domain\Terrain;
 use BattleForge\Domain\UnitState;
 use InvalidArgumentException;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
 final class MatchStateTest extends TestCase
@@ -24,6 +26,67 @@ final class MatchStateTest extends TestCase
         );
     }
 
+    public function testItRejectsAUnitOutsideTheBattlefield(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Unit alpha-1 is outside the battlefield.');
+
+        new MatchState(
+            new Battlefield(8, 8),
+            [$this->unit('alpha-1', 'alpha', new Position(8, 0))],
+            'alpha',
+        );
+    }
+
+    #[DataProvider('impassableTerrainProvider')]
+    public function testItRejectsALivingUnitOnImpassableTerrain(Terrain $terrain): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Living unit alpha-1 must stand on passable terrain.');
+
+        new MatchState(
+            new Battlefield(8, 8, ['0:0' => $terrain]),
+            [$this->unit('alpha-1', 'alpha')],
+            'alpha',
+        );
+    }
+
+    /**
+     * @return iterable<string, array{Terrain}>
+     */
+    public static function impassableTerrainProvider(): iterable
+    {
+        yield 'water' => [Terrain::Water];
+        yield 'blocking' => [Terrain::Blocking];
+    }
+
+    public function testItRejectsLivingUnitsAtTheSamePosition(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Living units cannot share position 0:0.');
+
+        new MatchState(
+            new Battlefield(8, 8),
+            [$this->unit('alpha-1', 'alpha'), $this->unit('bravo-1', 'bravo')],
+            'alpha',
+        );
+    }
+
+    public function testItAcceptsLivingAndDefeatedUnitsAtTheSamePosition(): void
+    {
+        $match = new MatchState(
+            new Battlefield(8, 8),
+            [
+                $this->unit('alpha-1', 'alpha'),
+                $this->unit('bravo-1', 'bravo', health: 0),
+            ],
+            'alpha',
+        );
+
+        self::assertSame('0:0', $match->unit('alpha-1')->position->key());
+        self::assertSame('0:0', $match->unit('bravo-1')->position->key());
+    }
+
     public function testItRejectsAnUnknownUnitLookup(): void
     {
         $match = $this->match();
@@ -169,14 +232,21 @@ final class MatchStateTest extends TestCase
     {
         return new MatchState(
             new Battlefield(8, 8),
-            [$this->unit('alpha-1', 'alpha'), $this->unit('bravo-1', 'bravo')],
+            [
+                $this->unit('alpha-1', 'alpha'),
+                $this->unit('bravo-1', 'bravo', new Position(7, 7)),
+            ],
             'alpha',
             actionLog: $actionLog,
         );
     }
 
-    private function unit(string $id, string $teamId): UnitState
-    {
-        return new UnitState($id, $teamId, new Position(0, 0), 10, 10, 4, 2, 4, 2);
+    private function unit(
+        string $id,
+        string $teamId,
+        ?Position $position = null,
+        int $health = 10,
+    ): UnitState {
+        return new UnitState($id, $teamId, $position ?? new Position(0, 0), 10, $health, 4, 2, 4, 2);
     }
 }