The weapon platform was a type of space station created by the Klingon Empire in the latter part of the 23rd century. It appeared as a massive ring structure with a large cannon present in the middle.
The platform was constructed during a short alliance between the Klingon government and the rogue Vulcan scientist T'Uerell. In exchange for certain minerals the Klingon Empire had mined, she had helped create a powerful beam weapon for the Klingons use against the United Federation of Planets. The weapon was being constructed in the Klingon held Kathra system and was still under construction at the time but it was hoped that it would be instrumental in defeating the Federation Starfleet.
Using a captured bird-of-prey with a new type of cloaking device that made it undetectable even to Klingon sensors, Starfleet forces—under direction of Captain James T. Kirk—were successful in gaining scans of the weapon as well as learn their alliance with T'Uerell. The information was presented to Starfleet Command who ordered an immediate offensive against the Klingons to prevent the completion of the weapon's platform.
A Starfleet strike force under the command of the USS Enterprise was then assembled which penetrated Klingon space and destroyed the weapon before it could be used against the Federation. (ST video game: Legacy)