One thing that many of you overlook on your treatises on Taxes
and Income is that in many if not most Medieval Kingdoms the
State had some sort of monopoly from which it derived a lot
of income.

Salt was one that comes to mind. Wool was another.
Silk was a state monopoly of Byzantium from ca 550 A.D.
and of course there was always Seignorage, the difference
between the cost of the metal in the coinage and the
stated value of the money itself.