You could make an "or" condition that is related to a hidden resource and assign that resource to certain cities.
Example:
Code:
c_gothic_armourer castle requires factions { northern_european, eastern_european, southern_european, } and event_counter GOTHIC_ARMOR 1 or hidden_resource hre
Then assign the "hre" resource to the Holy Roman Empire's starting cities, or just a few as you see fit. This way those cities would allow the creation of those buildings right away.
NOTE: Make sure you leave 2 black spaces before the word "or" because sometimes it doesn't work if you don't.