TomeNET

(4.6) Pits and vaults                                                           
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Usually monsters are found on their corresponding depth. You can browse the
file r_info in lib\game inside your TomeNET directory to find out which
monster occurs where. However there are some exceptions, where you can find
monsters out-of-depth, meaning that you will encounter way more powerful
enemies. That can be very dangerous but also very rewarding. Out-of-depth
monsters usually occur inside of so-called "pits" and "vaults". Pits are large
rooms, surrounded by a rectangle-shaped passage, containing monsters picked by
a pit-specific criterium (troll pit, zoo pit...).

Vaults are (sometimes extremely large) structures of rooms and passages, mostly
surrounded by permanent walls that cannot be damaged.

A vault can be a real adventure of its own! If your character isn't very
powerful compared to the dungeon level, you better stay out of vaults if their
structure doesn't allow easy monster killing.

Besides pits and vaults, there are also special rooms with features inside,
see 'Pit', 'Vault' or 'Room' in (0.4).