Thanks a lot for the kind comments and suggestions !
I think this game has potential to be expanded in many ways. Because I only had 4 days to work on it for this jam, the scope is very reduced at the moment. I originally wanted to have a much more detailed ship with realistic locations (control room, engine room, loading deck, infirmary, kitchen...). And of course, more items, weapons, enemies...
Regarding the stairs: They can only go up or down, not both ways. So when you go down from level 1 to 0, you end up in a place with no stairs (because you cannot go down anymore). So you have to walk to the location of the up-stairs. The opposite will happen on the highest level. This made sense to me when I coded it, but I see now that it can confuse players.