Gamzigrad, also known as Felix Romuliana, is an archaeological site located near the town of Zajecar in eastern Serbia. It is a well-preserved Late Roman palace and memorial complex dating back to the 3rd and 4th centuries AD.

The site was built by Emperor Galerius as a retirement palace for himself and his mother, Romula. It was named after his mother, whose name means "happy Romulus". The complex covers an area of about 10 hectares and consists of a palace, a temple, and a number of other buildings, as well as a mausoleum that was built for Galerius himself.

The palace was built in a rectangular shape and had a central courtyard with a number of buildings around it, including a large reception hall, living quarters, and a bathhouse. The temple was dedicated to the Roman goddess of hunting and the moon, Diana, and is one of the most significant buildings on the site. It is located in the northern part of the complex and has a rectangular floor plan with a semicircular apse on the eastern end.

The mausoleum is located outside the palace complex and is a monumental structure that stands about 35 meters high. It was built in the form of a rotunda and was decorated with beautiful mosaics and sculptures.

Gamzigrad was declared a UNESCO World Heritage Site in 2007 due to its exceptional historical and architectural value. It is considered one of the most significant and best-preserved examples of Roman architecture in Serbia and is a popular tourist destination for those interested in ancient history and architecture.