Are there any torches you can light with your magic powder? There might also be a bombable wall at the end of that middle corridor, I'm going off of memory.
There is an unopened chest in the west/middle part of the map? There's also a room with two torches you have to light with your magical rod that gives you a key. There is another key you can get by filling in the entire floor in that room where you move the machine around to fill in the tiles...