This fort is part of my supply line, placed between Constantinopole and Thessalonica...
... and this is how it may look during a siege if the enemy brought his artillery

edit:
On this screenshot you can see a ring of defensive forts around my capitol Antioch (it's not a greatest screenshot, it was created for some other purpose).
The function of this forts is not to defend the city, but to stop the invading army so I can engage it in the open field. When I built it, I was having a financial problems, earning only a few hundreds FL/turn. Then a crusading army came from nowhere, using it's doubled speed, and besieged Antioch. Since Antioch was my main economic city, I couldn't afford it to be besieged as I would lose it's income. That's why I built the forts around it.
They also proved usefull during the Timurid invasion...
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