Most fantasy role playing games use elements of the middle ages in their background ... but provide little or nothing in the way of real background ... and what they do provide is generally anachronistic or wrong.
Orbis Mundi is intended to bridge that gap.
The first chapter, Hard Times, covers Medieval and Feudal Government, Law and Justice.
The second chapter, Hard Money, deals with Money, Coinage, Prices, Economics and Guilds.
The third chapter, Hard Knocks, provides information on Medieval Life, Serfdom, Living Conditions, and Legal Status and Rights (including Women's rights).
The last chapter, The Market, is an extensive annotated Price List for goods likely to be available during the later middle ages, specifically the 14th and 15th centuries. From Alchemetical Supplies to Missile Weapons, from Armour to Libraries ... and much, much, more.
If you think your favourite FRPG has a background that is the most accurate representation of the middle ages SINCE the Middle Ages, then Orbis Mundi is not for you.
If you don't care about historical accuracy in even the slightest respect, then Orbis Mundi is not for you.
If you DO care about historical accuracy, and historically accurate prices and, for example, wish to understand why Longbows do NOT cost more than Longswords and much more, then Orbis Mundi IS for you.