Fixed placing more than one tower on a single cell

Assets/Prefabs/Terrain/HexCell.prefab

@@ -128,8 +128,6 @@ MonoBehaviour:
   coordinates:
     x: 0
     z: 0
-  isOccupied: 0
-  occupier: {fileID: 0}
   mesh: {fileID: 816763163768567949}
   pertubValue: 0.63529414
   materials:
@@ -145,3 +143,4 @@ MonoBehaviour:
   - {fileID: 0}
   - {fileID: 0}
   - {fileID: 0}
+  _occupyingObject: {fileID: 0}

Assets/Scripts/Player/PlayerStateController.cs

@@ -245,7 +245,7 @@ private void OnInteract()
 
         focusedInteractable.Interact(this); // interact with the current target
 
-        if (CurrentState == PlayerStates.BUILDING)
+        if (CurrentState == PlayerStates.BUILDING && !targetCell.IsOccupied)
         {
             // Build the turret we are holding
             focusedInteractable.GetComponent<TurretPrefabConstruction>().Construct(targetCell);

Assets/Scripts/Terrain/HexCell.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections;
 using UnityEngine;
 using Unity.Mathematics;
@@ -7,10 +8,6 @@ public class HexCell : MonoBehaviour {
 
 	public HexCoordinates coordinates;
 
-    // Is an object currently occupying (is placed on) the HexCell(tm)? This bit of code has the answers!
-    public bool isOccupied;
-    public GameObject occupier;
-
     [SerializeField]
     private MeshRenderer mesh;
 	//public RectTransform uiRect;
@@ -50,6 +47,34 @@ public Vector3 Position
 	[SerializeField]
 	HexCell[] neighbors;
 
+    // Is an object currently occupying (is placed on) the HexCell(tm)? This bit of code has the answers!
+    [SerializeField]
+    private GameObject _occupyingObject;
+
+    public GameObject OccupyingObject
+    {
+	    get => _occupyingObject;
+	    set
+	    {
+		    if (value && IsOccupied)
+		    {
+			    Destroy(value);
+			    throw new ArgumentException(
+				    $"Cannot place {value} on cell at {coordinates}, as it's already occupied by {_occupyingObject}!"
+			    );
+		    }
+			_occupyingObject = value;
+	    }
+    }
+
+    /// <returns>
+    /// Whether this cell has a game object placed on it or not.
+    /// <br/><br/>
+    /// (Destroying a game object that was occupying this cell, will make this property return `true`
+    /// - without having to change the value of `OccupyingObject`.)
+    /// </returns>
+    public bool IsOccupied => OccupyingObject;
+
 	private Transform hq;
 
     void Awake()
@@ -86,10 +111,4 @@ public HexEdgeType GetEdgeType(HexCell otherCell)
     {
 		return HexMetrics.GetEdgeType(elevation, otherCell.elevation);
     }
-
-    public void SetTower(GameObject tower)
-    {
-        isOccupied = true;
-        occupier = tower;
-    }
 }

Assets/Scripts/Towers/TurretPrefabConstruction.cs

@@ -8,8 +8,8 @@ public class TurretPrefabConstruction : Interactable
 
     public void Construct(HexCell targetCell)
     {
-        GameObject t = Instantiate(tower, targetCell.transform.position, Quaternion.identity);
-        targetCell.SetTower(t);
+        GameObject t = Instantiate(tower, targetCell.transform.position, tower.transform.rotation);
+        targetCell.OccupyingObject = t;
         Destroy(gameObject);
     }