Save game files are named slot_N.lua, where the N corresponds to the save game slot number. They can be backed up and restored, or copied to other computers like any normal file.
They are located in these directories:
C:\Documents and Settings\username\Application Data\game
|Windows 7/8/10 |
username: the name of the user you're using to login in the computer
game: kingdom_rush, kingdom_rush_origins or kingdom_rush_frontiers