I actually think part of the reason BTTD works so well is that it takes some of the concept from Gemcraft. They both allow you to build generic towers, and then "upgrade" them or add gems in gemcraft to give them different properties, and also allow you to combine the various properties to make mega mega towers, or multi color gems. BTTD is a bit simpler than gemcraft, without as many types and effects, and also is different in that its free moving rather than set path, but the tower upgrading and combining concept is a good one imo.
BTD might be easier than gemcraft but i'm pretty sure if you took vote most people would say BTD is more fun than gemcraft. I even have some reasons of my own:
1.You're monkeys! 2.They have various upgrades 3.They have various types and weapons 4.Great graphics 5.Addicting 6.Practically endless! 7.You can choose from hard, easy, or medium You can choose from many different tracks without having to unlock them 8.It's epic and action packed! 9.You're monkeys!
That might too many reasons but here's why it's better than GEMCRAFT (i'm going to show the faults of gemcraft):
1.You have to unlock stages before you can play them 2.No tower is unique except for it's gem 3.All towers do and have the exact same attack except for some effects 4.not that many different gems only about... 6 5.not that epic 6.(my opinion) gets boring after a while
Well as you can see i have many reasons why i think BTD is better than gemcraft heh-heh. So...yah that's what i think
Well I was comparing BTTD (bubble tanks tower defense) to gemcraft, but yeah :P. BTD is not really the same type imo, its a lot different. Its fun, but kinda in a diff way :P. Its not nearly as complicated though, and imo got a bit boring after a while, whereas both gemcraft and BTTD have a lot more customizability so they give me more stuff to do after I've beaten general lvls, plus both have less issues with lagging immensely (especially BTTD, I've had thousands of waves coming almost all at once with minimal lag).
I don't believe Creeper World hasn't been mentioned yet, I really liked that one despite how short it is. I found it challenging enough to keep my interest, and once you've got your footing down, rewarding enough to keep you playing.
Too bad I think you have to pay for anything more than the simulator, that's lame. :P