The Security Tower functioned as a detention center overseen by the Bespin Wing Guard, and it was a tower located in Cloud City which itself was on Bespin.
After the Battle of Hoth, Darth Vader seized control of Cloud City; subsequently, Baron Administrator Lando Calrissian, Lobot, along with a squad of Wing Guards, planned to ambush the stormtroopers who were guarding Leia Organa and Chewbacca. Following the confiscation of their weapons, these troopers were then transported to the Security Tower.