Those rooms filled with tons of monsters are called Dungeon Zoos and they can be a blessing or your worst nightmare. You'll get tons of experience if you can survive.

Once I was down to almost no hitpoints/spellpoints and hoping to find some food or drink in the next room. Then BAM! "DUNGEON ZOO". Poor Bran Stark (characters name) didn't have a chance after that.

In hindsight I should have passed a few turns and healed a little before opening any door.