One of the must sees in this often overlooked city. It is very close to the Triumphal Arch of Galerius and so there is no excuse for seeing one and not the other.
The Rotunda is not surprisingly round and dates from early times (about 4th century). It was built either as a temple or as a mausoleum to Galerius.
It's worth reading, at least a little, of the history of the building before visiting as that way they will appreciate it better.
Once inside we found it peaceful and away from the hustle and bustle of the city - it also helped that we visited when there were no organised trips visiting so I'd suggest you go during mid to late afternoon.
Admission charges are extremely cheap about 4 euro I seem to remember.More