The tomb (or memorial) of Galla Placidia was my favorite mosaic site, partly because it is small and the mosaics are relatively easy to see. I suggest that you read about Galla Placidia (https://en.wikipedia.org/wiki/Galla_Placidia) - she had quite a dramatic life, and must have been quite a person. (There should be an opera cycle about her - anybody want to write a libretto for me?) The building looks unremarkable from the outside, but inside it is like a fairyland. The pictures really can't render what it is like to be surrounded by jewels.