How the mines are laid out
The mines are divided into biomes, each starting at a fixed floor and running until the next begins. A biome sets what ore and gem appear, which artifacts can be dug up, what armour is themed to it, and how much combat experience its monsters give.
Floors are generated fresh each run, so nothing about a specific floor's layout is memorisable. What is stable is the biome: at a given depth you always know which materials are on the table.