Harlock said:
10-20% property tax is out-friggin-rageous, even for a fantasy setting. Of course, in a feudal system I can see this percentage and more for an income tax.
Out-friggin-rageous or, as we call it in the profession of economics, "confiscatory". The rental return on property has varied over the centuries, but is rarely less than 3% and rarely stays above 5% for long. So a 10-20%
ad valorem property tax amounts to a 200%-667% income tax on rental income. At least it does until the price of land falls through the floor. The baron had better fix his tax assessments to the market price, because if he insists on 10-20% of 'fair value' everyone will either abandon their land or else go bankrupt.
Mediaeval taxation is a vast and complex subject, as you will understand given that Mediaeval Europe spanned one thousand miles from Ireland to Muskovy, a thousand miles from Norway to Malta, and a thousand years from the fall to one Rome to the fall of the other (AD 476 to AD 1453). The only universals are that tax was always unsystematic, controversial, and it if ever yielded much it was soon crippling. {In England (a very wealthy country) the king was expected to live on the rents of his estates and the income of his courts. Taxes were imposed only as emergency measures. When there were taxes they tended to be things like 1/6 of all moveable property (one-off), or two shillings for every hearth, or to fall as duties on good imported or exported.}
The best readily-available source of information and suggestions that I can suggest is Adam Smith's "An inquiry into the nature and causes of the Wealth of Nations" [London, 1776]. Book III chapters ii-iii deals with economics under the feudal system, and Book V chapter ii deals with government revenues.
Regards,
Agback