I think everyone knows the concept of tower defense:
You have to stop several waves (usually 10-30), here 25 on medium. And the enboss(es), 1 on easy, 2 on medium and 4 on hard. If 30 units escape, the game ends, and you loose. If not, and you kill the endbosses too, you win.
The Attackers must be AI controlled, as the waves are scripted. And the Defenders must be player controlled.
There are 2 attackers: Tactical attacker, which uses few, but powerfull units and Stategic Attacker, he uses much, but weaker units.
And then, there are 2 defenders: Currently Named "Conventional Weapons Defender", Miniguns, Missiles etc, and "Advanced Weapons Defender", Railguns, EMP etc.
Here is some progress, only for the Conventional Weapons Defender:
First: The Minigun Emplacement. A light turret, armed with twin miniguns. Useable vs everything, but only effective vs Infantry and Light vehicles.

Next: The SSM Launcher. This one is a medium range artillery turret, fires 2 HE missiles, with quite a large AoE.

More: The FLAK cannon. This "little" double barreled, shrapnell grenades firing beast is a pure nightmare for aircraft.

And last, but truely not least, the heavy artillery. Firing barages 4 shells on enemy troops.
