Cenodrome is a small standalone map made in the mold of the TWHL Tower maps. Put together by regulars at The Whole Half-Life site, TWHL Tower was a collaborative mapping project where each mapper constructed a small floor of the tower, and each floor was joined to others by a stairway. Cenodrome is the long lost sub-basement of that tower.
Here is Cenodrome, the lost sub-basement of TWHL Tower.
Unzip the cenodrome folder to steamapps\common\half-life. Restart Steam and Cenodrome will appear in the Games list in your Library.