Make a Prison by Lawrence Block (Science Fiction Stories, January 1959) gets off to a pretty good start with two Alteans discussing a prisoner—the murderer of three of their kind, the first such crime in thirty generations—who is about to be imprisoned in a tall tower. They talk about the security precautions (the curved, unclimbable walls, the pneumatic delivery tubes, etc.) and then watch as the shackled prisoner is sent up to the accommodation at the top.
Several minutes later the prisoner throws his shackles down (the key was at the top of the tower), and then (spoiler) he climbs the rail and flies away.
This latter event broke the story for me as there is no build up to this surprising event—it just happens. I presume the twist might work for those who were assuming that the prisoner is a human.
* (Mediocre). 1,000 words. Story link.