Royal vaults are secret isolated rooms that can randomly appear in almost any normal level. They are disconnected from the rest of the dungeon, and must be entered by digging or teleporting in.
They can have the same sizes as any other regular room, and each tile contains some amount of gold lying on the ground, which is directly proportional to the level's DL. This makes their location easily detectable by using a scroll or wand of gold detection.
Royal vaults can be detected by the message "You hear the sound of falling coins" or "You hear the steps of the royal guard". Depending on the Listening and Perception (one of them must exceed 1d150 roll), the player may not see these messages. If the PC is wearing an amulet of greed while on a level where there is a royal vault, a message will occasionally appear depending on the B/U/C status of the amulet (see the corresponding article).
Each royal vault contains a royal guardian, which will protect all gold in the room — even that dropped by the PC or by monsters slain in the vault. If the PC picks up any gold, he will become hostile. Kicking gold out of the vault will also be met with hostility (though not in ADOM versions prior to r48).
Due to buggy behavior in ADOM versions prior to v. 1.2.0, royal guardians will not become hostile if any gold in the vault is picked up, but will start to attack if the PC just chats to them. It was fixed in later versions.