The Imperial Citadel was located on the planet Balstros.
After the Lord Emperor led the remnants of the defeated Sith to safe haven from the Federation and the Jedi, he proclaimed himself their Emperor and with his new found authority he promised to rebuild and restore the Sith Order to its former glory and more stronger than ever. Now the people loyal to him, they built him the citadel as an honor and a symbol of power to their supreme leader. The citadel was the tallest building in Clou City and stood at the heart of the city. The Citadel was known to house the Emperor and his most trusted inner circle and and was under the protection of the Elite Dark Guard.