Complex sentence: Clondalkin is believed to have been founded by Saint Mochua as a monastic settlement on the River Camac over 1,400 years ago; the round tower was built around a century later (circa 700 AD) as part of the monastery.
Split into simpler sentences: Clondalkin is believed to have been founded by Saint Mochua as a monastic settlement on the River Camac over 1,400 years ago. The round tower was built around a century later (circa 700 AD) as part of the monastery.