Alien Breed II is one of the rare amiga arcade games I've finished without cheatcodes. I have to admint that I spent hours on it I was 16 and I didn't always do my homework... I think it's hard but fair. In fact it's all about picking the right weapon at the right moment, and also you have to really know the maps by heart, pick the keys in the right order and use the computers to see the whole maps. The worst is the destruction sequence you really don't have time to get lost on the way to the exit.
I was never able to finish tower assault by the way. Alien Breed 3D I think I've almost finished it was ugly but fun. And the 3DII was much too difficult and too slow for me.
On Project X I also spent hours it is much more difficult I always died at the boss of the second level.