As with all TARDISes, it could travel in both time and space. Astrolabus used it to plunder many treasures from Earth's past. While Astrolabus was recovering Voyager's star charts, his TARDIS took the shape of a lighthouse. The Sixth Doctor followed Astrolabus back to it from Antarctica.
Once there, Astrolabus altered the Doctor's perception so the Time Lord thought he was hanging from the top of the tower when he was actually at the edge of a desk, with his feet touching the ground.
The Doctor realised the lighthouse was a TARDIS and tried to escape. He found his journey hindered by black tentacles that grabbed him as he passed, but managed to jump through the door. When the Doctor remembered where he had heard Astrolabus' name, identifying him as the Time Lord thief who had stolen The Book of the Old Time, he observed the TARDIS must have been stuffed full of the things Astrolabus plundered over the centuries. Astrolabus next turned his TARDIS into a rocket and left Earth, disappearing in an explosion. (COMIC: Voyager)