Which archeological structure in Saudi Arabia was also known as 'The Lonely Castle'?

The Qasr al-Farid (meaning ‘Lonely Castle’) is located in the archaeological site of Madâin Sâlih (known also as 'al-Hijr' or 'Hegra') in the north of Saudi Arabia. Although called a castle, the Qasr al-Farid was actually a tomb constructed around the 1st century AD. It is just one of the 111 monumental tombs scattered around the landscape of the Madâin Sâlih, a site which was inscribed by UNESCO (The United Nations Educational, Scientific and Cultural Organization ) as a World Heritage site in 2008. Of these tombs, 94 of them are decorated.

The Qasr al-Farid is one of the most famous tombs in Madâin Sâlih and was named as such due to the fact that it is completely isolated from the other tombs situated in the area. This is unusual, given that most of the monumental tombs in Madâin Sâlih were found to have been made in groups. These include the Qasr al-Bint tombs, the Qasr al-Sani tombs, and the tombs of the Jabal al-Mahjar area. The building of the Qasr al-Farid was actually never completed and it was never known for whom the tomb was built.