Whatever it is that you did, the Titans want revenge for it. And that means you must defend against them, with towers. Towers that whittle down their numbers as they swarm over the map, towards that final point of your homebase.
Each new Tower Defence game tends to have some sort of twist to set it apart from the others. However, Revenge of the Titans has focused on getting the basics right, forgoing the usual bells and whistles that serve to distract you from the fact that, really, you're just putting down some towers and waiting for bad guys to walk within range.
What does set it apart is the freedom you're allowed with how you set up your defences. Before you get to the fights themselves, there's an extensive research tree presented to you, allowing you to focus on where exactly you want to go, be it building better turrets, or passive bonuses like straight damage buffs.
Which then translates onto the battlefield, affording you a growing repertoire of toys to scatter across the map, using them in concert so blockades can force the Titans to bunch up before they break through and walk straight into a mine, wiping out five or six instead of the usual one. Or batteries and cooling towers, which boost your turret's magazine and firing rate, respectively, boosting damage output.
Add to all that various different Titan types, some armoured, some fast, some huge, some small, and you're going to have to mix things up regularly to keep them from breaching all the way to your command centre. One of your primary worries is researching yourself into a corner and being left with a useless loadout when facing the bigger boss Titans. Or, if you do focus entirely on the super powerful heavy turrets, you can't deal with swarms. The game favours a balanced approach, but this doesn't become readily apparent until you're already in trouble.
This is partly because the first five or six levels are easy to botch. You can build turrets in the Titan's path without much worry, have inefficient resource gathering and blow all your specials (which carry over from level to level), leaving you with poor tactics and poor equipment when you go up against something challenging. But, once you do overcome that obstacle, you'll be all the better to deal with the rest.
There are a pair of supplementary modes: Endless and Survival, two spins on surviving against increasingly large numbers of enemies. It's a nice distraction from the campaign, but by taking away the customisation of the research tree, they remain somewhat hollow.
While it has a tendency to lead you into a misstep with its choices, Revenge of the Titans is a game that rewards repeated playthroughs, so fine-tuning your research path and building placements are rewarding in and of themselves.