This is Porta Romana, just outside my apartment. In the old times, Milan city was surrounded by walls and big gates called "Porta" were set up on various sides of the wall for the citizens to enter. As the city grew, they would bring down the walls to allow the city to expand and build another wider wall, with other new "Porta"s. This hapened 3 times, so currently you can find the remains of 3 different walls in the city with beautiful, giant, ancient gates scattered througout the city.
Every day when I walk next to this, I am reminded I live next to ancient Roman ruins! And a sunset like this is the perfect decoration!