This level occupies the map slot MAP03. For other maps which occupy this slot, see Category:MAP03.
From the map start, drop down to the ground. Enter the staircase in the northwest of the building you just left. Walk through the fake wall with the lantern on it to find a hidden alcove (sector 302) with a soul sphere in it.
To the right of the alcove where the blue skull key is in, open the wall behind the candle to find an energy cell and a teleporter. Enter it to be teleported to the top of the fountain (sector 140) with the plasma gun on it.