Sentries

Turrets are unkillable, unmovable objects that sometimes spawn in world 8, also known as Mechtropolis

They can appear in a variety of colors, and usually spawn in the middle of the screen

Turrets attack by spinning 360 degrees, while shooting bullets at the rate of a rapid fire droid. These bullets can be dodged by jumping, getting behind shelter, or jumping onto the turret itself. After completing the stage, the turret will shut off, and will never be active again.