This is a stage from the Splatoon and Splatoon 2 games. Look closely for Judd and Li'l Judd. Judd will raise the flag with the color of the fighter or team currently in the lead.
Moray Towers has multiple floors to it, each at an angle. When the player runs towards a floor, they go on the downwards floor. The floors also act as soft platforms, allowing for multiple levels of fighting.
Right before the match starts, during the on-screen appearances, off-screen Inklings will fire ink at the stage, though this is solely for aesthetic purposes. When a player is in the lead, Judd and Li'l Judd will hold up the victory flags from Splatoon and Splatoon 2, with appropriate coloring for the player (red if Player 1, blue if Player 2, etc.), and will also get launched into the air when a player gets KO'd. If the music playing is Ink Me Up, Ebb & Flow, Muck Warfare, or Acid Hues, the stage will be at nighttime, just like how it appears in Splatfests.