Fun but could use work
The levels are rather short for a defense game. People are drawn to these games that like building up large defenses over a period of time, and making a new map every 3 small rounds really throws that off. I personally was put off knowing I only had a small handful of enemies to defend against before I had to start anew. I would suggest having longer levels (More rounds per map, or much longer rounds), and likewise having more upgrades per unit. Create more units that have different hit areas, like some throw a bomb forwards and hits a 2X2 square a square ahead of them, or a sniper that needs 5 blank squares in front of him before he can hit an enemy. This would create strategy on where to place units on the map, as well as adding to the possible length of each map, as people would not grow bored because they had to set up new unique defenders. The money is also an issue, you simply didn't earn enough to make it far. Defense games are about strategy and tactics, not banking. A game is not "hard" because you barely give the person money to work with, it is hard because they need to lean various tactics, manuevers and formations to beat various enemies. Limiting the money to such a degree is a cheap attempt at making it hard, and very annoying. I would also get rid of the slots, you almost never win, and the measily 100 dollars you do win makes it not even worth the time. I do like the digging idea though, perhaps let people find money, or special upgrades you can only get by exploring and digging. Make it clearer to the players on how exactly you earn bonus money.