Frankly I think the game is too hard to make it fun. You dont get the satisfaction of minless destruction, because its always on the verge of losing. Some people like it, I dont...
Plus the menu's are overly detailed and laggy. That is unnessary. Look at other games that have high rattings and you'd see they have very simplistic menus. And LARGE READABLE TEXT. Not saying yours is unreadable but I rather nice big icons, instead of squinting at some text
You should add
1)The ability to carry over your leveled units. Will add a rpg-ish aspect to your towers
2)Hot keys, it helps alot for most defence games
3)Magic spells are useless in this game cept trap. mp regens too slow, and the spells only effect 1 monster. Maybe BUFF spells for your characters. Or maybe a 1mp spell which lets the player click away himself, ect
4)The scenery is pointless. I rather find another empty spot then destroy a tree.
5)Digging for treasure is dumb. 1st, its random. 2nd, even if you get treasure, you have to play slots for gold.