At the northeast part of the map there are two round-shaped rooms, the eastern one needs the yellow key to open. Halfway, one of the walls very close to the door can be opened. This wall reveals a switch for a small lift which conceals a Megasphere and another switch. (sector 192)
The hidden switch mentioned above (the one hidden behind the megasphere) opens the northeast portion of the previous big roundish room (the one where you get the yellow skullkey), this portion has an invisibility sphere.