Value is set earlier.

src/engine/objects/Mob.java

@@ -1547,7 +1547,6 @@ public class Mob extends AbstractIntelligenceAgent {
                     break;
                 case GuardMinion:
                     this.agentType = AIAgentType.GUARDMINION;
-                    this.guardedCity = ZoneManager.getCityAtLocation(this.building.getLoc());
                     break;
                 case GuardWallArcher:
                     this.agentType = AIAgentType.GUARDWALLARCHER;