 The Pontcysyllte Aqueduct was built by Thomas Telford in 1805 and is the largest aqueduct in the United Kingdom ; it carries the Llangollen Canal over the River Dee and is a World Heritage Site , being considered a masterpiece of civil engineering . The Clwydian Range and Dee Valley constitute an Area of Outstanding Natural Beauty , one of just five in the whole of Wales . Denbigh , Colwyn and Ruthin are historic towns and Llangollen hosts the Llangollen International Musical Eisteddfod in July each year . 
