I like the concept, but a lot can be done to improve this game.
1. Upgrades for existing equipment. In addition to the regular units, the king should also be able to be upgraded.
2. Specialty towers. There is no reason to buy arrows or crossbowmen. If the arrows eventually got upgraded to include splash damage and the crossbowmen to incorporate penetration, there would be more use for them.
The user interface is very easy to use, but that comes at the cost of versatility. You cannot improve or sell units, nor can you repair any damage done to the 'guard'. This game is more of a shell of a tower defense / space invader clone. You can and should do much more with this game than you have.