Wallachia

According to the Fifth Doctor,  was, during at least the 15th century, a principality in what would become Romania by the 20th century. It bordered Transylvania. It was led by the Prince of Wallachia, who formally called it Ungro-Wallachia, and its capital city was Târgoviște.

In the 15th century, it was hotly contested between Christians, led by Dracula, and Muslims, led by the Turkish Sultan Mehmed II and aided by Radu the Handsome, Dracula's brother. (AUDIO: Son of the Dragon)

Behind the scenes
Technically, Wallachia is an ambiguous term, with several places in Slavic Europe bearing the name. However,, it is not believed that any DWU story takes place in any of the other known Wallachias, which makes the only known DWU Wallachia that of Ungro-Wallachia.

Even so, Ungro-Wallachia — or the Wallachia that forms the basis of modern Romania — is never really defined in the piece, except that it is said to border Transylvania.