Decent proof of concept for a game. Breaks horribly if you upgrade too far though.
I would say there should be extra requirements for prestige, like kill x enemies in a single attempt, increasing the required zone each time. For example:
1st Prestige: Donate 1000 blood
2nd Prestige: Donate 5000 blood and kill 70 enemies in the forest in a single run
3rd Prestige: Donate 20000 blood and kill 90 enemies in the desert in a single run
4th Prestige: Donate 80000 blood and kill 100 enemies in the mountains in a single run
And give more blood or another reward for increasing zone difficulty. Maybe give a bonus to a stat/reduce costs based on the highest wave cleared in each zone?
Last thing would be to increase the area size a bit. It is a bit cramped as it is. (Oh, and fix high levels causing you to just... die. When I got everything to level 60, the game almost froze, and as soon as a plant enemy showed up I just died about 10 seconds in.)