Answer
The answer is Ctesiphon, the capital of the Sasanian Empire. In 299 AD, Galerius, a Roman Caesar under Emperor Diocletian, launched a campaign against the Sasanians. After initial setbacks, Galerius managed to defeat the Sasanian king Narseh at the Battle of Satala. Following this victory, Galerius advanced deep into Sasanian territory and laid siege to Ctesiphon.