Agrivaina is fundamentally a historical setting. Yes...I take a bunch of different time periods, shove them together, and shake, but it's still historically based.
What I found really liberating is having something to fall back on as the base line. I don't need to flush out towns much because I basically know how a medieval village or city basic works (and even those work very differently form one another). Instead, I can put my create brain where it counts: how is the village DIFFERENT than a historical one.
I can also come up with interesting treasure pretty much on the fly. While my notes might say 250 gp, I'm going to te;; the party they found 27 silver denarii with the head of Trajen on one side and an Owl (symbol of god, Minerva) on the other worth 250 gp to the right buyer. It doesn't really matter than this coin didn't exist*, but that it COULD have been printed. See: liberation. So instead of thinking up interesting treasure beforehand, I can be busy creating more interesting magical items and adventure locations instead.
*It's a fusion of Trajan and a some later Roman coin that got passed around in class Friday. Can't remember which emperor it was under. I'm pretty sure he was dead, though.
I'm sure I could go on for a few pages like this, but you probably get the idea:
I'm using history as the background of the setting not because history is boring (and it isn't), but because it allows me to put my creative energies where I want to put them instead of spreading them thinly over everything else.