Thank you very much DrawMaster! I just uploaded a new version that should fix the issues you were having: the robot's engines are make noise so you can hear them behind doors now, and the doors have been totally reworked to have better cover.
Falling though the floor should also be much less likely now as I patched up a lot of holes.
There are now 3 difficulty options that do change things like reaction times and enemy bullet speeds. Easy is much easier than before, and hard is a fair jump in difficulty up, too.
Please give the new version a go and let me know what your thoughts are.