The surviving earthworks make more sense when set against the River Wye. Builth guarded a critical crossing between northern and southern Wales. In 1277 Edward I considered the surrounding country comparatively secure because local men were loyal and supplied a large share of his forces in Wales. Their loyalty gave the king a reason to protect their lands, so the crossing explains why a castle was useful even in friendly territory.
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Builth at the River Wye crossing
The opening places Builth Castle at a River Wye crossing between northern and southern Wales, where geography still explains the site despite the loss of its masonry.