7 Nights

In the beginning you have only 1 upgrade for each tower. You can unlock more by using imaginum on the upgrades screen and you can also unlock other useful stuff there (cat, bulldozer, soldiers).

Upgrades for towers:

?D? is for damage (3 levels)

?F? is for fire rate (3 levels)

?R? is for range (2 levels)

Also there is a tower specialty which first you have to unlock on the Upgrades screen then buy for each tower during the night.

Max upgrades for each tower: 9.