The Sonnenborgh is actually two museums in one: above is an exhibit on the history of astronomy in a site that was among the foremost astronomical research sites on earth in its heyday. Below are the foundations of the original bastions it's built upon. Though the exhibit could use an overhaul, this site remains special, perched on a hill overlooking the Singel.
On certain days you get a chance to look through the ancient telescopes at the sun (1-5pm Sun) and the moon, stars and planets (8-9.30pm Fri Apr-Sep; €10). For the latter you'll need to book online. There's also a mini-planetarium (based on the one in Franeker, Friesland) in the library.
