Located on the outskirts of the town, this remarkable structure is an exquisite example of early Indo-Islamic architecture. Designed by Abu Bakr of Herat, the mosque believed to have been built in two and a half days, and is thus named Adhai-din. According to another story the name is derived from a two and a half day fair held here every year. Built in II98 as the inscription indicates on the mihrab. This building was an important part of King Visaldeva`s kingdom. During the mughal period this was the only mosque in Ajmer. The name was given by fakirs who gathered here to celebrate the urs of Pir Panjaba Shah, which lasted two and half days. According to Jain tradition it was built by Seth Viramdeva kala for the Jain festival of Panch Kalyan mahotsava in 660AD. The foundation stone was laid by Jain Bhattarak Sri Viswanadji. Originally there was just one building and was used as a college house.

It is square 259ft on each side with cloisters on all four sides. It encloses a spacious courtyard and four splendid star shaped cloister towers on four corners, surmounted by magnificent chattris. The stone used is fine-grained pure silicious stone coloured yellow by iron. It originally stood on a high terrace with the back towards the hill. The temple of learning was on the western side and entrances on south and east. The interior consisted of a quadrangle 200ft x I75 ft. The college building is said to have been built in 53 AD by Visaldeva, the first Chauhan ruler. The cloisters extended 770ft now it is only 64ft left; it was destroyed by Ghuri`s attack on Ajmer in II92. Ghuri`s men added seven arches in front of the western side, and a mehrab and mimbar near it. The mehrab is of white marble and was added in II99 AD. The screen wall was added during the time of Illtumish in I2I3 AD. Daulat Rao Scindia was the first person to order for the monuments protection.

The three central arches are surrounded by 3 lines of writing, which are divided from each other by two bands of rich arabesque ornamentation. Two inner lines are Arabic and the outer line is square Tughra or Kufic in yellow limestone. The arches are said to have been constructed by Hindu masons and in Hindu style and design. It has a vast pillared hall 248ft x 40ft, flat recessed roof and has 9 octagonal compartments corresponding to the 7 arches. 5 rows of columns, one row placed against the rock wall, only 70 pillars remain standing and are in Hindu style.

There were two minars, the southern one has only one storey and the northern one has two and a part of horizontal belt writing containing the name of Sultan Iltutmish lies in the courtyard. Rich in inscription, six tablets were found in I875-6 made of polished basalt containing parts of an unknown play `Devi Vrigharaj` in Devnagari.