space permitting, it isn't cost effective to upgrade multi towers: every time you purchase a multi you get minimum 3 bullets hitting, each with power P, so for the cost X you get 3p. Upgrading multis initially costs you 2X for 2P , when for the same money you could get at least 6P buying new towers. (Granted this number goes down with higher upgrades, but it never nets positive.)
Upgrades need be worth more or cost less.
 "Zombotron"){kind=icon}
 "mini Tower Defence (mTD)"){kind=icon}