Was there a spawner stonehenge nearby maybe?

There's a structure that appears in the swamp that is a giant stone platform with henges (upright stones) around it. Inside it, in between the stones is usually three undead spawn piles. Draugr and skeletons. If you're near one of those they will just begin pouring out like a firehose.

The tip about using a hoe to level the ground that was posted above is... just... brilliant. I am dumbfounded by how brilliant that is. and I'm going to do that immediately to a few of the swamp areas I farm for resources.

The best suggestion is build some safe house throughout the swamp. if you have to, build raised walkways, or paths through trickier areas. Build raised stairs up the sides of the bigger non-destructible trees, so you have look out platforms.

And if I find any sunken crypts, I automatically build a little mini-fort on top of them, and build a portal inside. that way I can come and go while clearing out the crypt.