Wheres the balancing?
1. The game is very limited there is no variety in the levels whatsoever. All that changes from level to level is the number of arrows the castle shoots and its hitpoints.
2. Archers are so powerful they can win every level indefinitely with only about 20 of them if you repeatedly upgrade them. Cavalry are instant killed, while costing 10 times as much as infantry. Also ARCHERS HAVE LONGER RANGE THAN CANNONS!!! I mean seriuosly, whats up with that.
3. It involves no tactical skill at all. All you do is deploy units and you have no control over them.
Fix these and the game might be "OK" i would recommend making the castle send out units of its own.