The Chomsongdae is a small stone astronomical observatory dating from the 10th century — one of the oldest surviving structures of its kind in East Asia. It consists of five upright stone pillars supporting a flat slab, on which Koryo astronomers placed instruments to observe celestial movements and calculate the calendar.
It is a quiet, almost unassuming monument tucked away near the old palace grounds, easy to walk past if you don't know what it is. But it is one of the better physical reminders of the scientific sophistication of the Koryo court — the same era produced the world's first metal movable type, in nearby Cheongju. A short stop, but worth the few minutes to look properly.