A gate is the opening through which a room is entered. Every room is reached through at least one gate.
Sealed gates
If a room contains enemies, all of its gates are blocked by a vibrant purple field of magical energy, announced by a distinct sound. The room stays sealed until every enemy inside is killed, which includes anything summoned by an obelisk but not anything summoned by graves.
Hidden gates
Secret rooms are reached through gates that blend into the surrounding wall. See Secret rooms for how to spot and open them.
Compiled from what this wiki already says on Rooms, Floors, Secret rooms. No new facts were added.
Locked gates
Some gates carry a colour-coded lock, which is opened with a key of the matching colour found on the same floor. That is a different thing from the purple seal described above: a seal lifts when the room is cleared and cannot be opened with a key.
From the official manual
- The manual states the rule directly: rooms seal the moment you enter them, and to lift the seal you must defeat every enemy within.
The manual details above come from the official game manual, a primary source: the developer's own documentation rather than community research.