Complex sentence: Neither Monmouth nor its castle were attacked in the rebellion of Owain Glyndŵr in the early 1400s, although nearby Abergavenny and Grosmont were burned down in the uprising, but the unrest of the period did have a negative impact on the development of the town.
Split into simpler sentences: Neither Monmouth nor its castle were attacked in the rebellion of Owain Glyndŵr in the early 1400s, although nearby Abergavenny and Grosmont were burned down in the uprising. The unrest of the period did, however, have a negative impact on the development of the town.