Unlike early prison designs, capsules during Robotnik's rule had no visible ways to open them. Even so, they were weak enough to be destroyed with a Spin Attack.
Capsules, as they are known elsewhere, appeared at the end of every stage in many of the Sonic games on the Mega Drive. The capsule could be opened or outright destroyed by jumping on the large button on top. Such an opening device was a major weakness, allowing heroes to save the trapped citizens inside.