Pretty cool game. However I think the zombie spawn is kinda aggressive since the beginning . I know the purpose is to get as much score as possible, but since sun is gotten so slowly and peashooters don't actually seem to be able to fight a single zombie sometimes, I think the game is too hard.
One suggestion (if you are still working on this, of course). Would be that you add something like a "Wave" system.
Basically it would be a system that, instead of settting an unchangeable time for a zombie to spawn, it would start in something like 20+ seconds and for every zombie created, it would lessen 0.5 seconds on that time, plus every zombie destroyed would also do so.
Obviously that can get pretty out of hand at the late game, so maybe you could add something to make it that if the time for the spawn goes below 4 (or lower, if you wish a harder late game; or higher if you want it easier) it sets the time back to 4 or the number you chose.
That way, the 2nd zombie to spawn would do it after 19.5-19 seconds after the 1st zombie spawned, while the 21st zombie would do it after 4 or less seconds after the 20th zombie, basically increasing the zombie spawn as the game progresses. That would make it easier for the player to set up when starting the game but would ensure the game is not easy on the late game.
Hope this helps out! If this project isn't on development anymore, still I'd like to see what else you have to offer. This is pretty neat.