You could call the codes cheats, but they are built into the game. When you beat Survival mode, you unlock "Tower Mix" mode. When starting a new game, you'll have a check box for it. Check that box, and you can enter a 5 digit number that alters the towers in various ways. There are some Tower Mix codes posted in this thread, but there is also a thread specifically for them in the "Post Your Codes" section of the forums here.
FWIW, you might want to try a VR map, because you should get a lot more points on VR maps. A0003 is exactly the same as Basic, so you can play that the same way you play Basic, but you should get a lot more points per game.
The Titan shows up under certain specific conditions. I'll hide the details, in case you (or anyone else reading this) don't want to know.
He shows up if you're playing at difficulty 25 or higher, and have gone for 76 waves without losing any health. But waves 1 and 2 don't count. So, if you don't lose any health at all, or lose some in the first or second wave, he shows up in wave 79. If you lose some health in wave 3, and nothing after that, he shows up in wave 80. If you lose health in wave 23, and nothing after that, he shows up in wave 100. So, if it's after wave 23 and the Titan hasn't already come out to play, and you lose some health, you're not going to see him in that game.
As for something new or different, the basic game doesn't really change. There aren't any other enemies to find or anything like that. But there is Survival mode (which you apparently haven't beaten yet), and the other maps, including the VR maps, and Tower Mix mode. There are also a few towers that you can unlock (eg. teleports and mines), if you haven't done that yet.