This is the command tower as seen on the Devastator, an Imperial I-class Star Destroyer.
Command towers, also known as conning towers, were external structures housing the bridge on various Galactic Empire warships. The Imperial II-class Star Destroyer was one such Imperial vessel that sported a conning tower. During the Battle of Hoth, the conning tower of the Tyrant, an Imperial II-class Star Destroyer, was destroyed by an ion cannon blast fired from the Rebel Alliance's v-150 Planet Defender. Due to their vulnerability, the Resurgent-class Star Destroyer of the First Order did not incorporate conning towers into its design.
Imperial-class Star Destroyers' command towers were topped with either a tractor beam targeting array (on Imperial I-class Star Destroyers) or a communications tower (on Imperial II-class Star Destroyers and upgraded Imperial I-class vessels). Furthermore, two deflector shield generator globes were situated on either side of these structures.