Trade posts have a base cost of 150 to build and take 12 months. Base costs are modified by distance and local liege opinion. Each Patrician can only build a limited number of trade posts based upon the size of his Family Palace, the number of dynastic men he has in his court, and the level of Trade Practices technology in the republic's capital (not the patrician's capital).
Trade posts are owned by families within each Republic, and contiguous trade posts of the same family form Trade Zones. A trade post and a trade zone provide income to the owning family based on the wealth of the holdings and counties connected to the trade zone.
Republics with a trade post in a county have a 'Seize City' CB on the local city, subject to various constraints.
Feudal rulers with trade posts in their land receive an Embargo Casus Belli on the Republic, which if successful will raze the trade posts within their borders for a handsome payout. Rulers also have random event chains to attempt to destroy a trade post within their demesne or to demand a bribe from the post's family. Additionally, one Merchant Republic can offer monetary inducements to a Kingdom to embargo another Republic.
A trade post can be upgraded to increase taxes, trade value, or garrison and owner retinue.