Great game.
Those that are saying easy is too hard, you must really suck at tower defence games. Money was never a problem, you just have to know how to manage it right. By wave 25 on easy I had enogh money no never have to wait for what I needed. It's wave 33 now and I've got 1713 gold and don't need another tower.
Good game, I think the only real problem is the Pyromanic achievement, but I havn't looked into that myself.
As it says in the description, you think you're good at Tower Defence Games? Then you have to try this one.