RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Yellow key
The Old School RuneScape Wiki also has an article on Yellow key.

The Yellow key is obtained by killing a skeleton on the southern end of the second floor of Melzar's Maze. The key is used to open the south-western door.

RuneScape uses the British convention for floor numbering: ground floor, first floor (immediately above the ground floor), second floor, etc.