https://de.zxc.wiki/wiki/Schloss_Annenberg Schloss Anneberg in Tirol is a nice ‘adventure-sized’ castle, and its also notable because it had a library of about 250 volumes founded in 1470. This probably included a German translation of a Latin treatise on the subject of war from northern Italy circa 1300. A recent study calls it a translation of Vegetius, but the Latin treatise is bits of Vegetius and Aristotle and “I should talk about arms and armour here, but you know, the moderns have invented offensive and defensive arms which are much better than the ancients had, so just ask them.” So its more an original treatise heavily dependent on de re militari.
Regardless, it could inspire adventures around obtaining or defending rare books.