The Watchtower is the base of operations for Sentry and his wife Lindy Reynolds. It disappears and re-appears with Sentry's memory. Its most recent reappearance was when Sentry remembered who he was and joined the Avengers. Coincidentally, it appeared directly over the Avengers Tower, and alerted the public to the presence of the super team, prompting the new team of Avengers to announce themselves earlier than they had planned.
Until recently, it was believed that the Void was trapped within its basement. It turned out, however, that this was part of a ploy by Sentry to keep his insane alter ego satisfied and subdued.