The game is insanely hard, much too hard for a jam game... But, that aside, general idea sounds good, if it was a bit less strict, it would probably be a proper and fun defence game. The graphics look nice, the sounds are also nice. The core mechanics look properly implemented (timers for sending enemies and for shooting from towers, as well as the "economy" of buying towers), it's just the tuning of some values probably (it is basically impossible to get a positive score for a casual player like me during few tries in few minutes). I tried even the strategy of placing as many of cheapest defences as possible... Btw, there is a small ui bug - if you double click e.g. the grave placing button, it just subtracts 35 from currency, but does not place an object (or does, but in a weird place?). I only tried "level 1" multiple times btw. Looks like you could cook something together for the next jam if you all give yourself a bit time to playtest and tune the game :)

Hard to say if the game is good or not because I didn't know what to do, I was asked for attack/defense values with no context and then a series of menus that didn't seem to do anything? 
It could be the game misbehaving, I played this on Mac in case that matters