I played it, after the 3rd level (i think) this msg appeared:
___________________________________________
############################################################################################
FATAL ERROR in
action number 1
of Mouse Event for Left Button
for object object4:
Moving to next room after the last room.
at gml_Object_object4_Mouse_0
############################################################################################
--------------------------------------------------------------------------------------------
stack frame is
gml_Object_object4_Mouse_0 (line -1)
Dunno if this is the outro (lol) or something unwanted happened. :)
About the game: it was a challenge, short, not bad, but very barebone, as you said. With some pickups and extras and sounds, this could much better.