Information on the Arch of Constantine:

The arch of Constantine is a three-way arch that measures 21m in height, 27.7m in width, and 7.4m in depth. The central archway is 11.5m in height, and 6.5m in width, while the lateral archways are 7.4mx3.4m. The lower part, the arches, and supporting piers are built of white marble using the construction technique, Opus Quadratum. The attic is Opus Latericium covered with marble slabs.
The variation in construction techniques suggests different construction times for the two parts of the arch.

Spoila:

The decorative elements on the monument are from different periods and are generally considered to be spoila. The arch has elements from the reigns of Trajan, Hadrian, Marcus Aurelius, and Constantine. Some of the older elements of the Arch have been changed from the older emperors to show the face of Constantine. Much like how the Egyptians used parts from older pyramids to construct the newer ones.