Get inside the building with the double staircase leading nowhere. At the point where the stairs are level with each other, there is a candle on the ground in each side. Seen from the entry door (that slams shut behind you) tap the wall behind the right candle to get the secret, then tap the wall behind the left candle to get out again.

After you have taken the exit from #1, you are on a narrow ledge with unwelcome guests arriving. Cope with the gatecrashers, then jump over to the chainsaw on the red plint; that's the secret. From there, jump into the red box - that's the entrance to the 'Y' shaped building and the blue key.

Inside the building on the pic, there is some goodies and two teleporters. And somebody that don't like you using either. The left teleporter, as seen from the door, leads to ammo. The right, to a secret. That you will need immediately.

After all the rest of the level, return to secret #1 and take the exit. When you are back on the ledge, go to the right. Below you, a blue box. It is a tricky jump, but can be done. The inside of the box is the secret; when you push the skull switch, it's the exit to the secret level: E3M9 Warrens.