Arch of Constantine
The Arch of Constantine is the most imposing of all the triumphal arches in Rome. It was ordered by the Senate to recall the victory of Constantine over Maxentius.
Like the Arch of Septimius Severus, it has three openings and is along the street that celebrated all triumphs. It's completely covered with scenes that exalt emperor Constantine as reformer of the State, with some of the more glorious episodes of the Roman Empire.
But the peculiarity of the decorations of the arch are in the fact that the statues and reliefs that cover it were re-used from older monuments, creating a sort of puzzle of objects from different eras.
Maybe this was due to the cost and scarcity of skilled laborers or perhaps to the fact that almost all the artists were moving to Constantinople, the new capital of the Empire.
Then again, it may have been a conscious choice: the images had to illustrate the victories and power of Rome's new master through a visual language that the people were already familiar with so as to easily connect Constantine's reign with those of the most beloved emperors of the past.
Naturally, the faces on the sculptures and reliefs of the preceding emperors were changed to that of Constantine!
Since then, for similar reasons, it became normal to reuse decorations and elements from older buildings to embellish new constructions and in particular, churches.