This article had some interesting speculation about the location of Castle Corvinus, but I feel like much of it is disproved by Rise of the Lycans. Both the film and novelization mention Brașov, so it seems likely that the castle is within a few dozen miles of that city. The Underworld novelization makes it look like the castle is in the Alps instead, but I think that Rise of the Lycans supersedes that. As for the paragraph saying that the elders sailed from the castle to Buda after escaping, I think that the film and novelization contradict that: the novelization mentions sailing towards the Black Sea, and the film shows the ship entering a body of water much wider than the Danube.
Here is the part that I cut, in case anyone wants to read it or thinks it should be restored:
The novelization of Underworld shows the Vampires had titled the conflict resulting in the great blaze as "the Battle of the Alps", which has led to speculation that the castle is located near Triglav, the highest mountain in Slovenia and the highest peak of the Julian Alps, the closest coastal area being near modern day Monfalcone.
Since Hungary included most of what is now modern day Croatia, Slovakia and Slovenia during the 12th through the 15th century, the castle may possibly be located on that coast of the Adriatic Sea. Much of the Adriatic coastline is rocky, which would match with the appearance of the castle seen in Rise of the Lycans.
The novelization of Rise of the Lycans mentions that the Vampire Elder Viktor went to his estates near Buda after leaving the castle, which are located in modern day Budapest. This makes it possible that any Vampires from the castle could have taken the Slovenian river Soča further inland. The location of the river would place the castle somewhere near modern day Monfalcone. Buda is the highland half of today's Budapest, the capital of Hungary.