It's the best of times and the worst of times for tower defence games. While it's a fine thing to have no shortage of choice, the App Store, Kongregate et al being flooded with clones of clones of clones of clones risks diluting the raw appeal of building a bunch of big guns and watching them automatically decimate a marching army of morons.
So Revenge of the Titans being nominally a tower defence game is going to send a fair few potential fans running for the hills. But to lump Titans in with its legion of apparent genre-mates would be a terrible shame, as its raison d'être is to break tower defence down and rebuild it as something new, fresh and intense.
Let's start with a key difference. Rather than marching past your towers and getting blindly shot to buggery, the titular invading Titans will instead make a beeline for towers in their line of sight and do their very best to knock them down. This can be heartbreaking. In seconds, a carefully laid and horrifyingly expensive array of Multiblasters and Blast Cannons, supported by gizmos to decrease their reload time, increase their rate of fire and lengthen their detection radius, can be stomped into the ground by a horde of pixel-art alien fiends.
If this were a tower defence game, your structures would be invincible. If this were a real-time strategy game, you'd be able to move them out the way before death's pixellated grip closed round them. Titans is, essentially, the worst of both worlds: the fixedness of tower defence with the fragility of RTS. This is also why it's excellent.
This isn't a matter of building a neat little maze of turrets until the level ends. This is a game of constant cultivation, the large, open maps filled with several dozen entities that need your protection. In turn, your base the Titans' ultimate objective needs their protection, or more specifically their charmingly retro zappy lasers.
So you spend your time and your resources (earned by building similarly fragile refineries and from occasional drops) desperately trying to set up a ring of defences and then defending those defences: building, reinforcing, reloading, selling, redirecting, nervously spending one of the all-too-rare drops that can temporarily repair, protect or overclock your creations.
Hundreds of Titans will die, but still they come. As levels creep towards their end, the apparently endless spew of chomping 2D horrors finally peters out, the frown will lift from your face, the sweat will be wiped from your brow, and you will feel good.
Losses on your side are also inevitably and terribly heavy, although bonus cash-earning Medals can be won if you're one of those beautiful minds who can build a wall of death so efficient that nothing gets trashed. Once in a while, I did hit upon initial set-ups which proved brutally effective. The game can seem to be viciously cruel to its player, but that is partly because the openness of the maps and the build tree mask quite how mathematically precise Revenge of the Titans is at heart. The levels like seem like absolute chaos, big open landscapes for space invaders to swarm across but really they're filled with subtle choke points and knowing survival techniques.