Slot one is reserved on arty towers.

src/engine/objects/NPC.java

@@ -356,6 +356,12 @@ public class NPC extends AbstractCharacter {
 			if (slot == -1)
 				Logger.error("No available slot for NPC: " + this.getObjectUUID());
 
+			// Slot one is reserved on Arty towers for the trebuchet
+
+			if (building.getBlueprint() != null &&
+					building.getBlueprint().getBuildingGroup().equals(BuildingGroup.ARTYTOWER))
+				slot = slot + 1;
+
 			building.getHirelings().put(this, slot);
 
 			// Override bind and location for  this npc derived