diff --git a/src/micropolisj/build_tool/MakeTiles.java b/src/micropolisj/build_tool/MakeTiles.java
index 85d7078..061b9e3 100644
--- a/src/micropolisj/build_tool/MakeTiles.java
+++ b/src/micropolisj/build_tool/MakeTiles.java
@@ -310,9 +310,10 @@ public class MakeTiles
 
 		for (String layerStr : layerStrings) {
 
-			TileImageLayer rv = new TileImageLayer();
-			rv.below = result;
-			rv.above = parseLayerSpec(layerStr);
+			TileImageLayer rv = new TileImageLayer(
+				result,
+				parseLayerSpec(layerStr)
+				);
 			result = rv;
 		}
 
@@ -554,9 +555,10 @@ public class MakeTiles
 			String tagName = in.getLocalName();
 			if (tagName.equals("image")) {
 
-				TileImageLayer rv = new TileImageLayer();
-				rv.below = result;
-				rv.above = parseImageXml(in);
+				TileImageLayer rv = new TileImageLayer(
+					result,            //below
+					parseImageXml(in)  //above
+					);
 				result = rv;
 
 				skipToEndElement(in);
diff --git a/src/micropolisj/graphics/TileImage.java b/src/micropolisj/graphics/TileImage.java
index d437bab..f011b5f 100644
--- a/src/micropolisj/graphics/TileImage.java
+++ b/src/micropolisj/graphics/TileImage.java
@@ -22,8 +22,14 @@ public abstract class TileImage
 
 	public static class TileImageLayer extends TileImage
 	{
-		public TileImageLayer below;
-		public TileImage above;
+		public final TileImageLayer below;
+		public final TileImage above;
+
+		public TileImageLayer(TileImageLayer below, TileImage above)
+		{
+			this.below = below;
+			this.above = above;
+		}
 
 		@Override
 		public void drawWithTimeTo(Graphics2D gr, int time, int destX, int destY, int srcX, int srcY)