their damage according to their upgrade level.
their range increases over time by charging up electricity and loses all electricity when it shoots.(range increase should be the root of the charge, I know its math.)
charge speed upgrades according to their upgrade level.
max charge upgrades according to their upgrade level.
EDIT:
note that tesla coil is just the best fitting name, doesn't have to be that that tower gets removed.
Edited by Kyotouryuu, 02 March 2013 - 07:12 AM.