Some of the sounds have distortions or are harsh and should be improved. The music is a little too repetitive. For something atmospheric quickly, you could apply paulstretch to your existing music. Maybe post on /r/gamedev and see if an audio guy would be interested to help.
When I played I'd often fall down holes in rooms with with no monsters thinking I could go to a room below, or falling into a hole just before mobs spawned.
Maybe apply a hue shift, color map, or shader tint to your existing tiles to create some random color variations. You may even consider applying a gradient map type shader to the tiles layer and change that based on the zone.
On Windows, that extra black window is still a problem, but this is a Defold problem.