Is this considered a tower defense game?
i'd say
No. it appears that you "win" Bot Brigade by being
Offensive while the TD premise for the comp is:
"Protect or Defend against Waves of Adversaries via strategic placement of Towers or Obstacles"
otherwise, you could say
any game with a defensive aspect is Tower Defense, which most are not.
i'd still like to see bot brigade remade

(or similar, in AGK) just not for this comp.
meanwhile, please don't start another wave of "how about this? is this TD?" posts like the ones we staved off early on. read the requirements and if you have a question as to whether or not your idea "qualifies", feel free to PM
scraggle.