Complex sentence: This enabled the town to grow as a seaport but the need for additional defences was shown, when it was attacked by Welsh forces in 1187 and again in 1260 by Llewelyn the Great.
Split into simpler sentences: This enabled the town to grow as a seaport. But the need for additional defences was shown, when attacked by Welsh forces in 1187 and again in 1260 by Llewelyn the Great.