The Ponitrianske Museum is Nitra's main regional history museum, housed near the centre of the Old Town and covering archaeology, history, and natural history of the Nitra region from prehistoric times through the modern era. The archaeological collection is the highlight — this region has been continuously settled since the Neolithic, and the Great Moravian period material in particular is significant.
The museum won't take more than a couple of hours, but it provides useful context for the castle and the wider landscape of the region. Labelling is primarily in Slovak, though key pieces have English summaries.