Plympton Castle is a modest but worthwhile medieval survival, with earthworks marking the site of a Norman motte-and-bailey castle. There is not much masonry drama left, but the shape of the defences is still readable in the landscape.
It suits travellers who like ruins, local history, and small detours rather than polished attractions. Combine it with Plymptons old streets or nearby Dartmoor-edge excursions if you have transport.