Plain brick outside. Gold and glass inside.
Ravenna does not look like a great city. The streets are quiet and provincial, the buildings that matter are unremarkable brick from the outside, and several of them you would walk straight past.
Then you go in. Fifth- and sixth-century walls and vaults surfaced entirely in cut glass, set at deliberately uneven angles so that the gold moves as you move. Eight of these buildings are inscribed together as a UNESCO World Heritage site, and six of them are within a few flat streets of each other.
The reason it all survives is unglamorous: after 751 nobody here was rich enough to knock it down and start again.
4 days in Ravenna leaves room to slow right down.