I felt the humor of the original comic was depicted in the movie faithfully!
A ancient Roman architect named Lucius was led modern Japanese bath house, sentoh. There he got some ideas of Romen bath architecture, Thermae Romae.
He called modern Japanese flat-faced people. That way to call was very funny! Also Romen people were played these Japanese actors have distinct facial features, Hiroshi Abe, Kazuki Kitamura and Masachika Ichimura. So an actress of heroine couldn't find them in true European extras.
I felt like the ending was twisted. But I liked it because it got to the point.