Many of the permutations will be familiar to anyone who's sampled their fair share of tower defence titles: the general fodder, the quick attackers, the long-range units et al. There's also the Jackbot XL, a juddering behemoth of towering offensive capabilities that drops loads and loads of money when toppled.
Your turrets, then, are as much about blasting away the unending swathes of droids as they are for deterring any human Pros. Each turret can be upgraded two times, with level 3 encampments both costly and devastating. It all costs dollars, though, which are doled out for taking out opposing forces: you'll need to kill a lot of stuff to get your max out of your base, and a very skilful defence to keep it from being destroyed.
Money is also spent on upgrading your own personal set of abilities which, depending on which of the game's six Pro's (read: classes) you're playing as, offer up other, deeper avenues of tactical possibility. By far the most popular class at the moment is the Assassin, a lithe ninja with the ability to cloak, leap, backstab (which kills most characters instantly) and drop smoke bombs all over the place. They are, however, paper-thin. And you'll probably never get a chance to play as one if you're even remotely interested in tactically balancing your teams.
Assault and Tank fill out more general-purpose roles, roughly balancing offence and defence right down the middle. Assault, other than using the mandatory assault rifle, can ping out rather dastardly grenades with alarming regularity. The sturdier Tank has a laser that can set enemies on fire at short range, as well as fire precise long-range shots, and has a swinging melee attack that tears through everything in his path.
Like the Tank, the Gunner can deploy himself into a sturdy offensive fortress. Unlike the Tank, it's actually useful when the Gunner does it - firing out a steady steam of pain from a ghastly oversized minigun. He can also throw clumps of grenades - he doesn't mess about, basically. And he's slow.
Support acts as a mix of traditional Medic and Engineer classes. He can heal turrets and teammates, leech life from enemies, throw down his own personal turret, hack (friendly turrets for extra range, enemy turrets for a laugh) and chuck grenades that take days to explode but do massive damage.
And, finally, there's the Sniper. He snipes.
Each Pro's three skills - which, like turrets, can be upgraded with money - are mapped to similar face buttons, meaning switching from class to class is fairly cohesive and not a complete nightmare.