The Imperial Homeworld is a populated world, with cities and Annihilus' citadel. It's populated mostly by Arthrosians but also includes among its residents genetically engineered humanoid insectoids and representatives of various humanoid races serving Annihilus in the role of advisers, scientists, technicians, Centurions, etc. The world is heavily defended with legions of bugs and a space armada. During Annihilus' attempt to conquer the positive universe with an even more powerful Annihilation Wave, the imperial homeworld was turned into an interstellar worldship.
Which was equipped with planetary thrust engines, a global force field that prevented teleportation to the world by hostiles, and a surveillance system giving them the ability to spy on specific individuals throughout the positive universe. On the world was a teleportation gateway system capable of opening multiple gateways across the universe with pinpoint accuracy for their fleet and ground forces, and a planet-sized gateway for the planet itself. Which gave it the ability to travel from the Negative Zone to the positive universe and once in it to any location within the universe. It was escorted across the positive universe by a fleet of warships. The world was also turned into a planet destroyer, having the ability to shatter worlds with a single energy blast. The ability to destroy planets and open a gateway large enough for the planet was possible due to the harnessing of the captured Adam Warlock who possessed the power of a universe.
The Hulk was brought to the Imperial Homeworld by Pip the Troll in exchange for Heater Delight who was being held captive by Blastaar. Where Doctor Bultar used him to accelerate Annihilus' growth and boost his powers beyond what it used to be. Making him as physically powerful as the Hulk and giving him the ability to project fear. Thanos went to the planet and ended up fighting Annihilus, emerging victorious due to his greater experience, cunning, and intelligence though he soon left the world afterwards. Pip the Troll helped the Hulk escape the planet.
Afterwards, during Annihilus's attempt to conquer the positive universe with a second Annihilation Wave, Thanos and the Guardians of the Galaxy assaulted the planet which ended disastrously with Thanos ending up in a void without escape (forcing him to commit suicide) and the withdrawal of the Guardians of the Galaxy (who were forced to leave Pip behind). Annihilus ordered Doctor Bultar to take his planet into the positive universe. While in Death's realm, Thanos learned that Annihilus had used his mobile throneworld as a base of operations and planet destroyer in the positive universe. Slaying a number of cosmic beings (Galactus and Celestials), shattering and overthrowing interstellar powers, and destroying numerous worlds. All of which were reversed upon the resurrection of Thanos, which led to a final stand against the Annihilation Wave by Thanos and the resistance, and the annihilation of the universe by an awakened omnipotent Adam Warlock. Thanos then "convinced" the One-Above-Wall to restore the universe and both Thanos and Adam Warlock returned to Annihilus' throneworld before the war began, where Thanos led the Legions of the Dead and annihilated his army and thereby prevented the events from happening.