I'm curious to see how far the engine will have come in ACIII. Presumably Brotherhood and Revelations were built on a branched ACII engine, so I'm hopeful the ACIII could have more significant upgrades.
As for the architecture, cities like Philadelphia, Boston and New York are pretty old. There's some good engravings of Philly in 1800 here that give you an idea of what those cities were like in the period. And here's a big view of the Philadelphia skyline from a distance. Obviously not the ancient metropolises of Rome or Constantinople, but totally workable for the game's needs.
As for the architecture, cities like Philadelphia, Boston and New York are pretty old. There's some good engravings of Philly in 1800 here that give you an idea of what those cities were like in the period. And here's a big view of the Philadelphia skyline from a distance. Obviously not the ancient metropolises of Rome or Constantinople, but totally workable for the game's needs.