Inaccessible secret chamber on an exterior wall in Master
Levels map Bloodsea Keep (PC version and accurate ports).

When you first appear on this level, you'll be at the south
end of the map, and you'll be facing northeast.

This map is dominated by a single large, main structure.
Along the outside west wall, near the structure's northwest
corner, is a 5-sided protrusion.  This is the western half
of a large chamber, which contains one of the 4 "official"
(score-affecting) Secret areas on this map.

In Single-player and Cooperative-mode games, access to this
chamber depends on the death of a nearby Mancubus: When
that monster dies the walls surrounding that chamber drop
down, allowing access.

Note however that this monster only appears at Skill levels
3 and up, which means that this chamber will remain closed
in Single and Co-op games played at lower Skills.

(In the PS3/PSN version the chamber cannot be opened at any
time in Single-player or Co-op games, due to an error that
was introduced when porting the original PC map.)

In all versions of Bloodsea Keep, that area can be accessed
during Deathmatch games (at any Skill level) since a player
spawn point exists within the chamber, as well as a switch
that will open the exterior wall segments.

Click here to access a video and/or PC-compatible demo that
shows the location of all official Secrets on this level.