Watchtower I was a Wayne Enterprises item that was unknown to the Stock Holders as it was reffered to as a "line item hidden in the aerospace RND budget" by batman. It was a space station that orbitted earth for the league to watch over citiens of the world. It was the first Justice league's first Headquarter's.
After the Imperium invasion, General Wells commented that the Earth had been caught off-guard and did not have the means to detect advance warning of another alien invasion. Batman seized on this remark and contributed the Watchtower, which he called a "line item in the Wayne Industries R&D budget." In other words, it was a secret from Wayne's stockholders.
Since the Watchtower was also ideal for keeping a watch on the Earth below, the Justice League adopted it as their headquarters.
The Watchtower was almost fully automated and did not require a crew to operate and maintain it, beyond the seven League members. Entry was restricted to these seven, who also carried special transmitters to allow them entry.
While the League was in the Justice Lords' dimension, the Justice Lords took it upon themselves to arm the Watchtower with numerous weapons and close up vulnerable openings. This was all undone after the Justice League returned and defeated the Justice Lords.
The Watchtower was briefly occupied during the Thanagarian invasion, and later destroyed when Batman deliberately took it out of orbit and piloted it to an impact with the Thanagarians' hyperspace bypass generator.
The newly-expanded League was headquartered in the larger Watchtower II.
Alternate Timeline Edit
In an alternate future caused by the apparent death of Superman and the subsequent extinction of the human race, the Watchtower was left abandoned. Remarkably, according to Vandal Savage, it stayed in orbit for nearly 30,000 years, and most of its systems remained operational.