public Tower getTower(){
        int ID = (id-1) / 3;
        int level = (id-1) % 3 + 1;
        Tower t;
        if (ID == 0){           //air
            t = new AirTower();
        }else if (ID == 1){     //water
            t = new WaterTower();
        }else if (ID == 2){     //fire
            t = new FireTower();
        }else{                  //earth
            t = new EarthTower();
        }        
        for (int i = 1; i < level; i++){
            t.upgrade (true);
        }
        return t;
    }