The Space Pirate HQ was the unseen base of operations for the Space Pirates on the planet Talvania and possibly the Bermuda System. The facility was constructed underneath a massive underground Talvania Gas pocket, as the gas did not affect any of the Pirate personnel but would keep the Federation Force out.
Intelligence information gathered on the facility indicated that it was of massive size, housing perhaps hundreds or thousand of Pirates. As the facility was not directly accessible to the Galactic Federation, even with the new Mech technology due to the gas pocket, an alternative plan had been devised to destroy this complex.
The Galactic Federation deployed a massive energy Drill on the surface above the base to dig a direct access shaft to it and then drop a bomb through the shaft to destroy the complex. When the Space Pirates became aware of the Galactic Federation's attempt to dig directly towards their base they deployed various troops as well as well possibly salvaged Talvanian equipment such as Hover Bots and RB133 Auricrushers to destroy the drill. All these attacks were fended off by the Federation Force that stood guard over the drill while it was active.
The actual interior of the complex was never shown during the mission in which it was destroyed, M18: Hightower.