Spiegelslustturm stands on the Lahnberge east of Marburg, high enough to pull the whole town into view. From the top you can pick out the castle, Elisabethkirche, the university buildings, and the wooded ridges that make Marburg feel tucked into the valley rather than spread across it. It is a classic local viewpoint: simple, satisfying, and best on a clear day.
The walk up is part of the experience, though you can shorten the effort by using nearby roads and paths on the hill. It is worth saving for late afternoon, when the old town catches the light and the river valley is easier to read. The tower is not essential if you only have a few hours in Marburg, but it is a strong add-on once you have seen the historic centre.