A lot of technology issues are glossed over pretty thickly in fantasy stuff. Castles in D&D have towers and barbicans which would not be present with the availability of flying creatures. You see typical dark ages castles but these were refined and by the end of their days when powder came in castles went out. Some bizarre rounded ones with earth banks took their place but you never see any of these in campaigns.

Also hand held weapons made certain armour obsolete. Full plate became redundant in the latter period before powder as the late crossbows could penetrate a breast plate.

Given that there are magic and fantasic creatures in these settings then its anyones guess as to what might have really happened to castles and armour.