This is an underground base beneath the village that Reikon created. It holds enough rooms to house each member, a lounge for everyone to hang out, and for more mission purposes it has a dungeon and torture chamber. It is also big enough for a complete, "All purpose training field". He spent days on this underground training field, it stretches out past the village walls and a mile into the Oni village lands. You can have the room put you into any situation, even Bijuu situations. This is the same as his kage office. The entire training field is enchanted with a genjutsu. This allows for the "any situation" and also means more than one person can enter the training field at a time with any situation. For those who simply want to traning they can deny the genjutsu and simply train.. This eleminates space issues, since those using the genjutsu simulations wont actually be moving but standing still, while those not using it can run about the 5 mile underground training area. Also in the very center of the field there are bunkers to house each member comfortably. This is in case a member should decide to stay longer than a day.