A rune lock is a barrier that prevents you from entering a particular branch before you have a sufficient number of the runes of Zot. Three rune locks currently exist in Crawl:
- The entrance to The Vaults, which requires 1 rune to unlock;
- The entrances to Ziggurats, which require 2 runes to unlock; and
- The entrance to The Realm of Zot, which requires 3 runes to unlock. This serves as the main barrier preventing the player from simply descending as far as possible and reaching the Orb.
- In a standard game, only two runes are unavailable before entering the Vaults: the silver rune (found in the Vaults itself) and the golden rune (found in the Tomb, accessed through the Vaults via the Crypt). However, it is highly recommended to get the first rune from one of the two Lair branches (Snake Pit/Spider's Nest and Swamp/Shoals). Followers of Jiyva can easily acquire the rune from the Slime Pits once they have accumulated sufficient piety, but others will likely wish to wait until later.
- All runes are available before entering a Ziggurat (a portal to which is always found in the Depths) and the Realm of Zot. However, it is generally not a good idea to enter a Ziggurat with only two runes; all sorts of monsters including fiends and pandemonium lords can be found in there, and most characters with only two runes will not be prepared for them.
- Obtaining the second rune is generally best done in the other Lair branch from the list above. The third and final rune for opening the Realm of Zot is best obtained from either the Vaults, the Slime Pits, or the Abyss.
- All runes found in the Hells or Pandemonium are largely considered optional and are mainly used to improve the final score.
- A rune lock requiring 3 runes has always existed for the realm of Zot.
- In 0.19, the number of runes required to enter a Ziggurat was reduced from 3 to 2.
- In 0.14, the Vaults rune lock was added.
- Although the concept of a rune lock to the Vaults had been debated since very early versions of Crawl, it was only implemented after the 0.13 tournament, in which acquiring a rune before D:14 was an optional challenge conduct. Prior to the rune lock, players had little reason to explore the Lair branches until after they'd explored most of the main Dungeon and the Vaults, leaving them very overpowered for the challenges the branch endings provided.