Museum Director Mehmet Alkan said the castle has a magnificent appearance with its bastions, inner palace structure, a prayer room, and a dungeon.
"We have taken all security measures. We are waiting for our local and foreign visitors here," Alkan said.
Susanne Kalla, a visitor from Austria, said she could not visit the castle three years ago because it was closed to visitors.
"I am one of the luckiest people in the world to see this magnificent castle … I have seen many castles, but this place is very well preserved, I was very impressed," she said.
A French visitor, Emmanuel Juste, said he is a travel guide in his hometown and would add his impressions of the castle to his travel book and update it.
Mount Nemrut in Adiyaman province also houses the mausoleum of Antiochus I (69-34 BC), who reigned over Commagene, a kingdom founded north of Syria and the Euphrates after the collapse of Alexander's empire.
"It is one of the most ambitious constructions of the Hellenistic period," according to UNESCO.