[fph]

I don't play with tiles, but it looks like a vault (special room with most indestructible walls, dangerous monsters, and treasure) or a pit (rectangular room filled with treasure and monsters with a common theme, for instance all dragons, all undead, animals...)
In both cases, be *very* careful --- good loot comes with dangerous monsters. You'd better detect monsters to know what is expecting (if you can), and be ready to bail out and leave the level while your HP is still in the positive.