As far as size, Ctesiphon became a major city of the ancient world. It had started as merely a suburb of the nearby and much larger Seleucia, the grand seat of the post-Alexander Seleucid dynasty. Ctesiphon was reportedly founded on the opposite bank of the Tigris as a western bastion and a winter retreat of the Parthians, deliberately separate from the Greek town across the river. It was, along with Seleucia, in an important position to oversee trade flowing from the Iranian heartland towards Anatolia and the Mediterranean (Strabo 16.1.6). From the middle of the 1st century BCE under the ruler Orodes II, Ctesiphon became the "capital" of the Parthian empire, or at least the most important administrative urban center in the western portion of their vast territory. Ammianus Marcellinus writes that the city greatly expanded under Pacorus I (ca. 39 BCE) and was encircled by walls of some sort (23.6.23). In the next few centuries, it both grew in extent and importance (apparently) and also became the constant target of Roman aggression. It was taken by Roman armies several times during the Imperial period.

Very little is known about the features of the city, though most seem to agree that it was vast, probably merged with Seleucia into one greater contiguous metropolitan area. Despite a good deal of modern excavation, the specifics remain elusive. German, American, and Italian efforts in the 20th century have investigated several areas, both at Seleucia and across the river at what must have been Ctesiphon. There was a third focal point, supposedly built under Vologeses I (1st c. CE), called Vologesocerta, which was an economically-focused enclave of some sort. It's location is still a matter for debate. One problem is that the Tigris has shifted in her banks since antiquity, and now possibly bisects the old site of Ctesiphon. This German plan, which seems to be modern or at least fairly recent (but I don't know the exact source), sites Vologesocerta on the south end of the metropolitan area, and also shows the shifting course of the Tigris to the east.

No known features or monuments from the ancient period are known. The famous ruin known from the site today is from Late Antiquity. I am not sure why so little is known from the site, given its rather robust excavation history, and I'm unable to access any excavation publications on it (and they'd be in pre-war German anyway--yuck). I can hazard a guess that the whole site is probably under a considerable amount of Tigris flood deposits which perhaps makes ground survey difficult. I don't know for sure, though.