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The debate went on, voices rising, until Flitwick put everyone's minds to rest. To his credit, Professor Flitwick knew more about the construction of Hogwarts than what Professor Bagshot had consigned in her masterpiece, and could deliver a better history lesson than Professor Cuthbert Binns ever had, before or after his death.

It turned out that Hogwarts had first been built, according to the Architect's plans, as a mostly non-magical castle, though some of the stonework had bene Transfigured rather than carved. None Conjured thought.

Well—

There had been a Conjured tower, known simply as the Conjuration Tower, which Professor Slytherin used as his laboratory; but a few months after the Founders had begun to live in the Castle, Slytherin, as part of his experiments, had done something that had launched the entire Conjuration Tower up into the night sky like an anachronistic rocket. Calculations indicated that the Tower had probably crashed somewhere on the Planet Mars, and, clearly quite intent on retrieving whatever was inside the Tower, Slytherin had built the Astronomy Tower where the Conjuration Tower had once stood, using it to scry into space, hoping to get a glimpse of his lost haunts and work out a plan to retrieve them. The plan was abandoned after Slytherin's exile, and, to make use of the damned Astronomy Tower, they had created the Astronomy Class.

http://www.fanfiction.net/s/12682621/62/