This "sigil ledge" (if you look at the automap, it's the west side) has an invisible wall blocking it. That's fine.
(http://xhatahx.weebly.com/uploads/6/0/1/8/6018829/screenshot-doom-20161002-190613_orig.png)
This "sigil ledge" on the other hand (east), can be accessed. And if you land down on it, you'll die from "falling", even though obviously there's a ledge under you.
(http://xhatahx.weebly.com/uploads/6/0/1/8/6018829/screenshot-doom-20161002-190603_orig.png)