The Weir Gate was a semi-permanent portal between Tamriel and the Slipstream Realm in which the Battlespire Academy was later established. It was known to have existed since at least 2E 582. Once the Pillar Of Light (the Tamriel side of the portal) was entered, the user would arrive at the Weir Gate in the Battlespire.
In 3E 398, during the Imperial Simulacrum, the Weir Gate was sealed by Mehrunes Dagon's forces with a powerful sigil in the shape of the Daedric letter "Hefhed". This prevented the Imperial Legion reinforcements from entering the Weir Gate, and so the battlemages were sealed within.
Years after Dagon's invasion was defeated, the mage Morian Zenas and his apprentice used the Weir Gate to access the shattered remains of the academy. Despite an extensive search through the ruins, they discovered that all links to Oblivion had been broken.