[Radek]

You should be able to kill the mephits inside the "cages". If you aren't sure whether you target the right location, then save the game, cheat in an invisibility scroll or a potion and send an explorer to check what should be targeted. The potion is POTN10, the scrolls are SCRL1J or SCRL1Y. Then restart the game. If your explorer cannot find anything to target then something is really wrong with your game. Check the bug reports and patches, visit Baldurdash, or reinstall. I am playing an unpatched game and I've never had problems with the cages.
The cages are the key to this part of the dungeon. Ignore the mephits around and kill the mephits inside the cages as fast as possible. Then deal with the rest of the mephits. Otherwise, the cages will produce new mephits infinitely. Fighting them without putting the cages out of work is futile.
AFAIK, there is no key to the door leading out of the room with cages in the game. The "key" is killing the portals. The door unlocks after killing them.