@Neyjour: Those are a pair of large stone arches that act as planar gates, actually. One of them goes to the normal world; the other goes to the primal world of the fey. This particular in-between realm formed as a result a botched ritual that was supposed to repurpose the gate from the normal world for use in interplanetary transport instead of interplanar transport. Things went badly. Now it's got a giant swampland attached to it full of corrupted fey, bog mummies, and assorted primal beings of earth and water. Lethlin Mire is the seventh and final such realm of this campaign, though they've each had a different theme (decadent city-state, desert continent, shadowy realm of vampires, tropical island, steampunk city, howling void of emptiness, and now a swamp).

It's a little sad, actually. I've been working on this campaign for four years now, and the end is in sight.

@hireshmontvellos: That's very good feedback. If I have time I may tweak it some more, but it's more likely that I'll just take that into account the next time I make a swamp. I've got four sessions for four distinct campaigns in the next six days, and two of those I'm GM'ing, so time is in short supply. I've been doing GM prep like a mad man all week.