For the record, BlazingPhoenix managed to identify it on IRC -- it's the outdoor secret area of E1M3, or more specifically, the "evil mirror world" version of it as part of an extensive secret area I added. Try flipping the first screenshot horizontally and the resemblance will hit you. :P
I decided to give the same treatment to a couple of E4 maps ("Sepulcher" and "Shattered Bridge," namely), and they turned out not so bad. Just simple retextures this time around, but it's amazing how much a new paint job can change the look and feel of a map.
No new sectors were added in the process of making these save for the new scene at E4M4's entrance. Not bad for not adding detail, eh?