The original Watchtower was a space station orbiting the Earth that served as the headquarters of the original Justice League.
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 largely an automated facility and did not require any extensive 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.
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 even though he disrupted the gravity of the solar system and most of its systems remained operational.
In the DC comics, the Justice League's Watchtower was a base stationed on the moon. The Watchtower here is more similar to the Justice League Satellite that replaced the Watchtower.