Weird effect where sprites get left behind = Hall of Mirrors. HOM for short. It's a bug that happens when you have a single-sided linedef without a texture.
Also, I'd recommend reading the Doom Builder tutorial on the main site. You need a thok barrier, learn how to add end level sectors... just many, many, things you are missing. It's a good start, a noob would often quit on first try, but you just need to find out how to develop SRB2-esque levels, not DooM levels.
I have a feeling turrets don't shoot because of one-sided linedefs (can't see through the one-sided linedef, so no shoot). That can be fixed by using two-sided linedefs. But then again, just start with the tutorial.