The Watchtower was the headquarters of the the Justice League. A general term, the Watchtower initially applied to a satellite that orbited the Earth which was utilized by the League after they abandoned the Hall of Justice.
The original Watchtower satellite housed Boom Tube technology and had computerized records of the League's history since they began to utilize the Watchtower. It also contained a meeting room and several storage rooms. After an attack on the League by Despero, the Watchtower was brought down to Earth. The original Watchtower was later destroyed by the Crime Syndicate.
The second Watchtower was provided to the Justice League by Lex Luthor as a replacement for the destroyed satellite. It was extremely similar in design and function to the original satellite. This Watchtower was later abandoned by the League in favor for their third base of operations. Soon after it was taken over by Brother Eye who moved to the surface of the moon and transformed it into a command base. The ruins of the second Watchtower were destroyed during the Battle of Neo Gotham along with Brother Eye and Inque.
The third Watchtower was created by the Justice League and was an actual tower on Earth as opposed to a satellite. Created while the second Watchtower was still active, the third Watchtower was located in Metropolis and survived the Eye War despite the world around it being turned into the Eye World. However, after the Eye War began this Watchtower was abandoned.
After Brother Eye was defeated Matt McGinnis left the safety of Gotham City and headed to Metropolis with John Stewart's Green Lantern Ring in hopes of finding the Watchtower.After eventually reaching the remains of the Watchtower a battle between Tim Drake and the newly revealed Splicers. The Green Lantern Ring activated and led Matt into a hidden chamber, where he found all of the Justice League members Superman, Warhawk, Big Barda, Aquagirl and Kai-Ro all in stasis.