The Siege of Atlantis was a battle in the Second Olympian War. The Titan Army, which was led by Oceanus, attacked Atlantis, in an effort to overthrow Poseidon. This was part of the Titan plan to split the Gods, to prevent an Olympian victory. The plan ultimately failed when Poseidon, under the urging of Percy Jackson, chose to attack Typhon in New York. From what we know it wasn't really a siege but a battle by Poseidon's Palace.