This was really cool, I loved the concept. Two things that I noticed:
I'm not sure whether this is already the case, but the pricing of certain fields relative to how useful they are was a bit off in some places. For example, fields that can only be activated on high numbers (e.g. 27) are extremely situational because you can only use them if you didn't buy anything in the round before and almost always end up basically being single use only because you can't get above their counter value ever again, whereas a field that activates on, lets say 15, can easily be activated multiple times throughout because it is vastly more flexible.
Another thing is that sometimes, ranged enemies just don't move in a way that lets you even hit them before the time runs out, this almost happened multiple times and I'd imagine losing that way would be very frustrating.
Also, you can kinda cheese by leaving time pickups for next round, they don't despawn. not sure if that's intentional.