The mini-map is the map overlay on the HUD, opened with the map key (Tab by default).
What it shows
As each room is entered it is revealed on the mini-map, coloured according to the type of room, and the display also indicates whether that room contains a key. Fountain rooms show in green.
Seeing more
The Eye Ring carries a Secret vision effect, which displays orange door markers on both the map and the mini-map, marking the hidden doors into secret rooms.
Compiled from what this wiki already says on Rooms, Eye Ring, Secret rooms. No new facts were added.
From the official manual
- The manual lists the map overlay as one of the six elements of the game screen, and gives Tab as the default key for View Map.
The manual details above come from the official game manual, a primary source: the developer's own documentation rather than community research.