I don't regard the fort as an exploit, I regard it as a "trading camp" It's the one thing that actually makes the game playable as HRE for me, as without this I would never be able to build up an economy, defend my lands, build up an offensive military etc...

As soon as I can, I go to Timbuktu, build a fort on 1 of the gold resources and fill it with merchants, right now in my HRE campaign I have 15 merchants trading gold in Timbuktu, and still I have some turns where I don't have money and my building queues (spelling?) are stalled. Having to garrison your city's because the AI keeps planting spies, and having to garrison your crusader kingdom because people are unhappy over religion and the numerous Egyptian spies in the cities is a huge drain on your economy, not to mention the fact you have to build build build and recruit recruit recruit to defend from Jihads and prepare for the mongol and Timurid invasions.

not to mention the money you need to keep the pope happy so he won't interfere in your wars in Italy