The Coliseum Ruins in The Legend of Zelda: Breath of the Wild is a landmark where you can defeat waves of enemies. You can find the Coliseum to the north of The Great Plateau.
The Coliseum causes monsters to rank up as you progress in your adventure. The landmark will also summon some very powerful opponents later on, such as the Silver Lynel (the hardest enemy to defeat in the game).
Also, you can find two treasure chests at the top of the ruins.