Mausoleum of Augustus
This seemingly indestructible pile of bricks has been here for 2,000 years and will probably remain for another 2,000. Like the larger tomb of Hadrian across the river, this was once a circular marble-covered affair with tall cypresses, symmetrical groupings of Egyptian obelisks, and some of Europe's most spectacular ornamentation.
The Mausoleum is the Tomb of Augustus and of the principal members of his family, the gens Julia-Claudia, and as such was an extremely important building in the ancient city. The last Roman emperor to be buried here was Nerva in 98 AD.
The imperial remains stayed intact here until the V century, when invading barbarians smashed the bronze gates and stole the golden urns, emptying the ashes on the ground outside. You can't enter, but you can walk along the four streets encircling it, now being renovated.