Very little is known of the Citadel of Antiquity, other than it is where the 10 remaining S'pht clans made their final stand against the ever-powerful Pfhor during the war between the two species over 1000 years ago. This was the conclusive battle which led to the end of the war. When arriving here, Durandal tells you that the defenses are still online and therefore access to the primary tower is impossible.
Durandal also informs you that the main entrance to the tower is surrounded by a moat (whether this is part of the tower or whether it was put there by the S'pht is debateable).
Durandal also makes note as to how the final S'pht died during the final hours of the S'pht-Pfhor war. "The Pfhor appear to have been reluctant to attack the Citadel directly. Radiation lingering in the area even after a thousand years suggests that they irradiated the Citadel and let the S'pht die slowly.