Cost per tile: 150
Health per tile: 1250

 Monster Attracts:                  Size Needed:                        Also needs:
   Demon Spawn                           1 tile                                     Lair
        Orc                                         9 tiles                              Barracks

The training room is where most of your minions will spend most of their time,  though many would rather be researching or manufacturing. It's good to have working creatures spend at least part of their time training, so swap them around  so that they spend some of their time doing their primary job and some of their time training. It won't help you if a powerful party storms your walls and all you  have to defend your Heart is a handful of level one Warlocks.
On the other hand, training can eat up your gold reserves like popcorn. If you're on a tight budget, place some doors on the training room entrances and lock them. Put your creatures somewhere where they can be of use, and wait until you build up a healthy nest egg before you open the doors again.
 The training room is also a good defensive guard against breaches in your dungeon - place training rooms near possible weak points (water or areas that have been mined) so that any intruders will stumble upon a horde of your minions immediately