If your map has no light entities in it, the map will be fullbright. As soon as you add even one light entity, all the non-illuminated areas will become dark. 

Thanks! I had a feeling it would be something stupidly simple / obvious... 

Is there a way to make one specific room in a map look fullbright other than stuffing it with maxed out lights? I had this idea of making a funny "noob mapper" secret with blocky offgrid architecture and texture misalignments aplenty... 

Is your friend.

Check out my SM185 entry for a good use case. 

 

Make sure you use the exit texture on a wall. The entire wall. And columns that have gaps under them not touching the floor. And a door that flickers through a wall when it's open. 

Thanks for the ideas, Qmaster! 

If you want more inspiration, play SM49. The theme was "crappy
Maps"

https://www.quaddicted.com/reviews/sm49_pack.html 

A whole slew of crappy maps? I'm just making one secret room, guys... ;) Thanks anyway metl, I'll check it out.