Deca Tower
From Inkipedia, the Splatoon wiki
Deca Tower is the central tower in Inkopolis Square in Splatoon 2. Similar to Inkopolis Tower in Splatoon, it serves as the game's Battle Lobby.
Once inside the Deca Tower, you'll be able to play Turf War, Private Battle, join friends, and play Ranked Battle if reached level 10. You can also play League Battle once you have reached the B- rank in any Ranked game mode.