This city's architect, Martinego, sent in a request asking to investigate an ancient fortress. He's a well-know architect who worked on the fortification of the town of Rhodes. But at the same time, he also shows interest in ancient architecture. I suppose those with such abilities are the earnest learners.
It is said that there was a fortress in the Kingdom of Parthia, which whould not fall no matter how many times the Roman Empire, in its prime, had attacked. Once you understand the structure of this fortress, it'll help you build a stronger fortress. A history scholar in Rome should know something about this. Go and see him to hear what he has to say.
A Fortress in Dessert
Rome - History Scholar near the ruins outside the Coliseum
Oh, you must be talking about the fortress that is said to have been in the north of Baghdad. Records say that it wouldn't fall despite being attacked by the famed Roman emperors such as Trajan. If you head to the upper Euphrates area, you may be able to discover it.
It is said that this fortress had such a strong structure that it withstood repeated attacks by the Roman Empire led by notable emperors such as Trajan. If you search in the Upper Euphrates, you might be able to discover the ruins. Investigate there and report the results.