This line is probably just a random direction the writers chose, but geographically speaking, it was the Ottoman Empire that lay south of Wallachia. At the time Castlevania takes place, the Ottomans (chiefly Islam) were very open and tolerant of other religions compared to their European neighbors and granted asylum to cultural minorities. So not only is Trevor’s suggestion of heading south a means of escaping Dracula’s Horde, it’s also a means for Sypha’s people to escape religious persecution.
Sypha’s grandfather refers to their people as the Codrii Speakers. The Romanian word codru means ‘forest’ and codrii means ‘woods.’ So Sypha’s people are woodland Speakers.
By the time Vlad III Dracula ascended the throne for the second time in August 1456, a comet had been sighted over Wallachia. Based on the description by observing astronomers at the time and the calculations of moving celestial bodies, it is believed this was Halley’s Comet. The historical Dracula interpreted the sighting as a good omen and later incorporated the famous comet in coins minted during his reign. (Google searches show me none of the historical Dracula’s coins survive today, if he minted any specific to his reign at all. My source for this one is a book published in 1989: Dracula, Prince of Many Faces by Florescu and McNally, pg. 83. So make of this one what you will.)
The capital of Wallachia moved around a few times. When the principality was first founded in 1310, it was located at Curtea de Argeș. During the majority of Vlad III Dracula’s lifetime, it was at Târgoviște. In the autumn of 1476, it is believed he moved the capital to Bucharest for economic and military purposes and it remains to be the capital of Romania today. (Plus, in the context of the series, Târgoviște was a ruined city by 1476, so no wonder the ruling government cleared out.)