Dark_Falcon: I was Wikiwalking earlier and after looking at castles & fortresses and then going in pursuit of more info about the Mongol invasion of Europe, my stroll led me to a site that I think you and others here who like military history will find interesting (if you’re not already familiar with it), so I thought I’d pass it along. From the home page:

De Re Militari
^{The Society for Medieval Military History}

De Re Militari, begun in 1992, is the Society for the Study of Medieval Military History. Here we host many primary sources, articles, dissertations, and resources for the study of military actions, technology, and topics from the fall of Rome to early seventeenth century. We also have an active online Book Reviews section. We are the principle online source for scholarly information about warfare in the Middle Ages. Every year, and DRM sponsors lectures at the International Congress for Medieval Studies (Kalamazoo, MI), the International Medieval Congress (Leeds, UK), and the Society for Military History (peripatetic). […]

deremilitari.org

They had lots of interesting looking stuff on the Mongols, and I also saw some stuff on the Muslims & the Crusades that looked pretty neat.