Quote Originally Posted by Tyberius View Post
Building prerequisites:

Trading post: Fort
Merchant: Keep, Trading post
Merchant's guild: Castle, Merchant
Master Merchant: Citadel, Merchant's Guild

The main prerequisite to build the trading post chain is that any tradable goods must be present on the province
All factions can build the whole chain in all periods.
Thank you for the response. I finally figured it out last night. I tried the Papacy 'Late era', and took Naples from the rebels which does have tradable goods.