Nice job getting in, but look out for those guards. Muggshot's got those machine-gun toting dogs hunting like a pack.
― Bentley warning Sly about the guards inside the Boneyard Casino[src]
Upon entering the area, Sly had to flip a switch to open the door. Once inside, Bentley warned him to watch out for the machine gun-toting guards, who were patrolling the area like a pack. Sly had to fight his way through several of these goons to get to the treasure key.
When Sly found his way to the end of the area, he came across a giant spinning roulette wheel, with the treasure key stashed inside the glass in the center. Sly needed to jump along the wheel in the opposite direction of its rotation to hit the green key spot. After hitting it three times, the glass dome in the center broke, allowing Sly to retrieve the key.
Man, I'm getting swift with deciphering these codes. Dial 3-3-8.