given the book it is from was a translation of a work by Procopius (who did not illustrate his writings), and was published in 1888, the medieval appearance was probably intentional on the part of the artist. that was fairly common back then. few people took the time to understand how historical sites would have actually looked, and they tended to depict them with medieval elements as if that were a universal style rather than something that was developed fairly recently. (of course part of that is the gothic style was having a bit of a revival at the time)
i'd use crenelations and texture details closer to the walls of constantinople
Having worked at Durham Cathedral (also responsible for Durham Castle) I don't think the diagrams look very Gothic at all. Maybe a little.
I'm still weighing up all the info I've been sent and reading I have to do; so it will be some time before I come to a decision.