How does one gain access to Colossus Boulevard?

You have to defeat almost all the bosses in Wizard City

Defeat the Harvest Lord, the Kraken and Sergeant Skullsplitter (all of whom are located in Triton Avenue).

Complete Cyclops Lane by defeating General Akilles and Eyus Maximus.

Vanquish all enemies located in Firecat Alley (Melweena Smite, Bastila Gravewynd, Alicane Swiftarrow and/or Wormguts).

Venture to Olde Town and defeat Foulgaze in the field of battle.

Obtain 750 crowns or purchase a game membership. Without one of the two, you cannot enter Colossus Boulevard.

Speak to Merle and he will send you on a quest to find Private O'Doyle. The private has the key to Colossus Boulevard.

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