It's a walkway in Ravnica that's used by people of all guilds. It even has different guild symbols in the pillars!
In fact, if you look closely, you can see that people from multiple guilds are actually walking on the promenade right now! This person is definitely from the Izzet League.
And over on the left, there's someone from the Rakdos Cult talking to someone from the Selesnya Conclave!