Onderland is the setting for my Spellcraft & Swordplay campaign. Onderland is a fantastical version of Anglo-Saxon Britain during the Heptarchy. It is a land of invasions and acculturation; of misty moors and ominous meres; of a people coming out of a dark age and building a new culture on the ruins of those they conquered. It is the Age of Lords, when no one can truly claim supreme power, but many wish to.
Onderland is not, however, Anglo-Saxon Britain. Aside from the obvious differences between fantasy and reality, there are other differences. Onderland is land-locked. Although the huge Cobalt Sea touches it in the West, there are no oceans. As a result, the Ondermen are not notable as sailors. Instead, they are famed horsemen.
Onderland is a sandbox setting for old-school play. The Anglo-Saxon elements are there for colour. This isn't a history lesson. Whenever the early medieval flavour is at odds with game-play, the game wins out.