This is a beautifully restored structure that you can can spend hours looking at. Make sure to investigate all the doors and climb up to the top to see all the views. The museums inside should be a point of interest. The painting gallery is great and admission there gets you into all the museums. The porcelain gallery is also amazing. I did not love the mathematical gallery--mostly old (1400s) and pretty math instruments (rulers, compasses, some clocks). Not worth 6 euros on its own but pay 10 and go to all and it's worth a stop in.
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