The King's Castle, or Zamek Królewski, stands above Poznan's old town on the site of the medieval residence begun by Duke Przemysł I and expanded by King Przemysł II. Much of what you see today is reconstruction, but the site still matters because it marks Poznan's role in early Polish royal history.
The tower is the main reason to climb up: from here you get a good view over the Old Market Square, the church towers, and the compact historic centre.