The Circle object is commonly used for collision and as a placeholder graphic in game development. Circles can be added to entities which implement the ICollidable interface to act as the entity's collision shape.
To add a circle to an existing entity:
Expand the entity
Right-click on the entity's Objects folder
Select Add Object
Verify the FlatRedBall or Custom Type option is selected
Select Circle
Click OK
Circles can also be added to a new entity. To do this:
Right-click on Entities
Select Add Entity
Check the Circle option. Notice that this automatically checks the ICollidable check box.