Sun temple (Keshavark) at Ujjain is present in the campus of Kaliyadeh mahal (North most corner of city Ujjain).
Ujjain map showing Sun temple
In the olden days, king Arjuna learned Astra vidya from Indra. After that, Arjuna kílled Nivata Kavacha rakshasas as Guru dakshina. Indra was pleased very much. He asked Arjuna to take a boon from him. Arjuna asked the idols of Lord Sun from him. They are previously worshiped by Indra to get glory. Indra gave them.
Lord Sri Krishna knew this. He told Arjuna that those idols are very powerful and they should be consecrated(Done pratishta) in Ujjain. The idol placed by Sri Krishna is called as Keshavaraka (Keshavaditya) and the idol placed by Arjuna is called as Nararka (Nraditya).
Naraditya is present near Kal bhairav temple.
Renu teertha is present near Keshavarka.
Brahma kundam is also present in campus, which is the place where Lord Brahma did Yagna in the ancient days.
Every year Nara deep yatra will be conducted from Naraditya to Keshavaditya.
Worshiping these idols will give glory, fame and power.