Drone is a recently-ish released game by David W Wilson, free of charge.
As Jayisgames.com explained it quite well, I'll just let that part to them:
A hybrid tower defense shooter game, Drone puts you in the role of a little movable machine with a lot of potential. As you move through each level, you must take out waves of foes in real-time using various weapons purchased through the shop. You can also buy stationary machines that will fend off enemy attacks, collect money for you, or spawn helper dudes that will go forth and do your dirty work. An excellent mixing of genres with just the right level of difficulty, and with hi-def visuals, ten weapons, ten buildings, three sets of armor, and 16 enemy types, you'll get several hours of entertainment at minimum.
With a small download of only 10 MB, it's very much worth it. A great timewaster and pretty deep gameplay in one.
Media. (linked due to large images)
"A juggernaut tank shooting at an axebot, base on the right"
"Burning some axebots with a flamethrower, while taking fire from a machinegunner."
Download and more information.