Pretty good but IF your castle is made of blocks, the enemies should be too. shoot one and try to make it fall apart. Just an idea though, you do this your way.
I dont know shit about game designing, but i think it needs
-Enemies
-a story perhaps
-no offence, but a better menu
-a health bar
-treads on your tank
Note: plz dont b offended! this game is WAY better then anything i could have created!