I would say intermediate for something like this. First person shooters (especially ones like Cthon, which could be extrapolated to a top-down shooter and still essentially play the same) are relatively easy compared to something like a real-time strategy game (super-hard), but they're more difficult that say a top-down/side scrolling shooter or puzzle game. The hardest parts of Cthon were making the player movement feel good; I did a lot of testing and tweaking to make sure collision detection felt natural and the field of view was ample but didn't cause motion sickness.