Neidpath Castle is a compact 14th-century tower house on a wooded bend of the River Tweed, just west of Peebles. Its strength is the setting: stone walls, river below, trees around it, and a footpath from town that makes the approach feel properly Border country.
The castle is not always open like a standard museum, so check access before planning around the interior. Even from outside, it is a fine target for a riverside walk.