How to Build an Enemy Spawn Trigger in Unity (C#)

In this tutorial, you’ll build a simple enemy spawning system in Unity. When the player enters a trigger, enemies appear at designated spawn points. The player can remove them by touching them, and the trigger keeps spawning enemies until it reaches a specified total.

Prerequisites

• Unity 6.6 (6000.6.8f1)
• Basic familiarity with the Unity Editor (Hierarchy, Inspector, and Project windows)
• The free First Person + Third Person | Character Controllers package (also known as the Starter Assets) from Unity Technologies

Set up the project

1. Create a new project

   In Unity Hub, create a new project and choose the Universal 3D template.

   

2. Check the empty project

   This is what an empty project looks like.

   

Add a floor

1. Create a plane

   Go to GameObject > 3D Object > Plane.

   

2. Reset the transform

   In the Inspector, right-click the Transform header and choose Reset. This puts the floor at the origin.

   

3. Enlarge the floor

   Set the Scale to (5, 1, 5). A Unity plane is 10 × 10 meters by default, so this gives you a 50 × 50 meter floor.

   

Import the Starter Assets

1. Open the Asset Store tab

   Click the Asset Store tab to activate it.

   

2. Go to the Asset Store website

   Click Search online to open the Asset Store.

   

3. Search for the Starter Assets

   Enter Starter Assets Third Person in the search bar at the top, then click the magnifying glass icon.

   

4. Select the package

   Click First Person + Third Person | Character Controllers from Unity Technologies.

   

5. Add it to your assets

   Click Add to My Assets.

   

6. Open the Package Manager

   Back in Unity, open the Package Manager with Window > Package Management > Package Manager.

   

7. Download the package

   Under My Assets, select First Person + Third Person | Character Controllers and click Download.

   

8. Import the package

   When the download finishes, click Import 2.0.2 to project. Your version number may differ.

   

9. Confirm the files

   In the Import Unity Package window, leave everything checked and click Import.

   

10. Install the dependencies

    When asked to install or upgrade dependencies, click Install/Upgrade. If Unity asks to restart the editor, click Yes.

    

Add the player

1. Open the Prefabs folder

   In the Project window, go to Assets/Starter Assets/Runtime/ThirdPersonController/Prefabs.

   

2. Drag the player into the scene

   Drag NestedParentArmature_Unpack from the Project window into the Hierarchy.

   

3. Delete the original Main Camera

   Select the Main Camera that came with the template, the one at the top of the Hierarchy, and press Delete or select Delete from the right-click menu. The prefab brings its own camera, and two cameras would conflict.

   

4. Select the player

   In the Hierarchy, click the arrow next to NestedParentArmature_Unpack to expand it, then select PlayerArmature.

   

5. Center the player

   In the Inspector, set its Position to (0, 0, 0).

   

   The parent object is a container that groups the player and its cameras. The character itself is PlayerArmature, so that’s the object to position.

6. Test the player

   Click the Game tab, then press Play. Use WASD to move, the mouse to look around, Space to jump, and Shift to sprint.

   

   If you click inside the Game view, the mouse cursor is hidden and locked. Press Esc to get it back.

   

   If the character falls through the floor, stop Play mode and set the Y position of PlayerArmature to 0.1.

7. Save the scene

   While the game runs, the Play button becomes a Stop button. Click it to stop the game, then press Ctrl + S to save the scene. Changes made during Play mode are lost when you stop, so make any adjustments after you exit.

   

Create the enemy

1. Make folders for your files

   Make a SpawnTutorial folder with Materials, Prefabs, and Scripts inside to keep your work separate from the Starter Assets.

   

2. Create the capsule

   First you create the enemy in the scene. Later, you turn it into a prefab.

   1. Go to GameObject > 3D Object > Capsule to create a capsule for the enemy. A white capsule appears in the Scene view and in the Hierarchy as Capsule.

   2. Rename it to Enemy.

      

3. Make the enemy red

   Give the enemy a simple red material to mark it as hostile.

   1. In the Project window, open SpawnTutorial/Materials.
   2. Right-click in the empty area and choose Create > Material. In some Unity 6 versions, it’s under Create > Rendering > Material.

   3. Rename it EnemyRed.

      

   4. With EnemyRed selected, find Base Map in the Inspector. It has a small color box on the right.

      

   5. Click the color box and pick red. You can type FF0000 into the Hexadecimal field for pure red, or choose a slightly darker red like CC0000, which looks less harsh under lighting.

      

   6. Close the color picker.

   7. Apply the material to the capsule.

      Drag EnemyRed from the Project window onto the capsule, either in the Scene view or on Enemy in the Hierarchy. The capsule turns red.

      The capsule may sit partly in the floor. That's fine, since spawned enemies get their position from spawn points.

      

4. Make the collider a trigger

   A normal collider is solid, so the player would bump into the capsule. A trigger collider lets the player walk into it, and Unity sends an OnTriggerEnter message that your script will use to remove the enemy.

   1. Select Enemy in the Hierarchy.

   2. In the Capsule Collider component, check Is Trigger.

      

5. Turn it into a prefab

   A prefab is a reusable template saved in your project. The spawner will create copies of it at runtime, so the enemy in the scene is no longer needed once the prefab exists.

   1. Drag Enemy from the Hierarchy into the SpawnTutorial/Prefabs folder in the Project window.

      

   2. Select Enemy in the Hierarchy and press Delete.

6. Check the player's tag

   The Enemy script identifies the player by its tag, so PlayerArmature needs the Player tag. The Starter Assets already set this tag, but it’s worth confirming.

   1. Select PlayerArmature in the Hierarchy.

   2. In the Inspector, confirm Tag is set to Player. If it isn't, choose Player from the drop-down.

      

7. Create the Enemy script

   The enemy removes itself when the player touches it. Keeping this logic on the enemy means you don't have to modify the Starter Assets player.

   1. In SpawnTutorial/Scripts, right-click and choose Create > Scripting > MonoBehaviour Script. Name it Enemy.

   2. Double-click Enemy to open it in your code editor, and replace its contents with this code:
      

      using UnityEngine; public class Enemy : MonoBehaviour { private void OnTriggerEnter(Collider other) { if (other.CompareTag("Player")) { Destroy(gameObject); } } } 

      Save the file and switch back to Unity. Unity recompiles the script automatically.

   3. Select the Enemy prefab in SpawnTutorial/Prefabs. In the Inspector, click Add Component and choose Enemy.

      

   How it works:

- Unity calls `OnTriggerEnter` when another collider enters the enemy's trigger. The player's Character Controller counts as a collider, so no Rigidbody is needed. - `CompareTag("Player")` checks whether the object that entered is the player. It's faster than comparing `other.tag == "Player"`. - `Destroy(gameObject)` removes the whole enemy. Writing `Destroy(this)` instead would remove only the script and leave the capsule in the scene. 

1. Test the enemy

   1. Drag the Enemy prefab into the Scene view a few meters in front of the player. Unity places it on the floor.
   2. Press Play and walk into the capsule. It disappears.

   3. Stop Play mode and delete the test enemy from the scene.

      

Create the spawn trigger

The spawn trigger is a box marked by a see-through orange visual. When the player walks into it, the box disappears and enemies spawn at a set of spawn points, keeping no more than a set number alive at once until the trigger has spawned its total.

1. Create the SpawnGroupTrigger script

   1. In SpawnTutorial/Scripts, create a new MonoBehaviour Script named SpawnGroupTrigger.

   2. Double-click it to open it in your code editor, and replace its contents with this code:
      

      using System.Collections.Generic; using UnityEngine; using UnityEngine.Events; [RequireComponent(typeof(BoxCollider))] public class SpawnGroupTrigger : MonoBehaviour { [SerializeField] private GameObject enemyPrefab; [SerializeField] private Transform[] spawnPoints; [SerializeField, Min(1)] private int totalToSpawn = 6; [SerializeField, Min(1)] private int maxAlive = 3; [SerializeField, Min(0f)] private float minDistanceFromPlayer = 3f; [SerializeField] private bool disableAfterUse = true; [SerializeField] private UnityEvent onTriggered; private readonly List<GameObject> aliveEnemies = new List<GameObject>(); private Transform player; private int spawnedCount; private bool isSpawning; private bool hasTriggered; // Called when the component is first added. Makes the box a trigger automatically. private void Reset() { GetComponent<BoxCollider>().isTrigger = true; } private void OnTriggerEnter(Collider other) { if (!other.CompareTag("Player")) return; if (isSpawning) return; if (disableAfterUse && hasTriggered) return; if (enemyPrefab == null || spawnPoints == null || spawnPoints.Length == 0) { Debug.LogWarning($"{name}: assign an enemy prefab and at least one spawn point.", this); return; } player = other.transform; hasTriggered = true; isSpawning = true; spawnedCount = 0; onTriggered?.Invoke(); } private void Update() { if (!isSpawning) return; // Destroyed objects compare equal to null in Unity, so this drops enemies the player removed. aliveEnemies.RemoveAll(enemy => enemy == null); while (aliveEnemies.Count < maxAlive && spawnedCount < totalToSpawn) { Transform point = FindFreeSpawnPoint(); if (point == null) break; // No usable point right now. Try again next frame. SpawnEnemy(point); } if (spawnedCount >= totalToSpawn) { isSpawning = false; } } // Returns a spawn point that has no enemy on it and isn't too close to the player. private Transform FindFreeSpawnPoint() { for (int i = 0; i < spawnPoints.Length; i++) { // Start at a different point each time so enemies spread out. Transform point = spawnPoints[(spawnedCount + i) % spawnPoints.Length]; if (point == null) continue; if (player != null && Vector3.Distance(point.position, player.position) < minDistanceFromPlayer) continue; if (IsOccupied(point)) continue; return point; } return null; } private bool IsOccupied(Transform point) { foreach (GameObject enemy in aliveEnemies) { if (enemy != null && Vector3.Distance(enemy.transform.position, point.position) < 0.5f) { return true; } } return false; } private void SpawnEnemy(Transform point) { GameObject enemy = Instantiate(enemyPrefab, point.position, point.rotation); aliveEnemies.Add(enemy); spawnedCount++; } // Draws the trigger box and spawn points in the Scene view so you can see them while editing. private void OnDrawGizmos() { BoxCollider box = GetComponent<BoxCollider>(); if (box != null) { Gizmos.color = new Color(1f, 0.6f, 0f); Gizmos.matrix = transform.localToWorldMatrix; Gizmos.DrawWireCube(box.center, box.size); Gizmos.matrix = Matrix4x4.identity; } if (spawnPoints == null) return; Gizmos.color = Color.red; foreach (Transform point in spawnPoints) { if (point == null) continue; Gizmos.DrawSphere(point.position, 0.3f); Gizmos.DrawLine(transform.position, point.position); } } } 

      Save the file and switch back to Unity.

   How it works:

- **Settings:** `[SerializeField]` makes each private field editable in the Inspector. `[Min(1)]` stops the Inspector from accepting zero or negative counts. - **`Reset`:** Unity calls this when the component is first added. It checks **Is Trigger** on the Box Collider for you, and `[RequireComponent]` makes sure the Box Collider exists. - **`OnTriggerEnter`:** starts spawning when the player enters. It ignores every...