I've seen a couple bugs, and one error, so I'll just list them here:
On Hell difficulty, I keep getting victory before all the enemies are dead, it's usually only 1-3 left so this may be intended?
Hell 1-4 doesn't actually unlock 1-5, cleared it twice, it crashed before the 3rd, with the following error:
___________________________________________
############################################################################################
ERROR in
action number 1
of Step Event0
for object obj_dilf_penetrate_Goblin:
Unable to find instance for object index 103737
at gml_Object_obj_dilf_penetrate_Goblin_Step_0
############################################################################################
gml_Object_obj_dilf_penetrate_Goblin_Step_0 (line -1)