camps are evident. There was a Roman fort and amphitheatre at Tomen y Mur. Roads are known to have connected with Segontium (Caernarfon) and Deva Victrix (Chester) and include the northern reaches of Sarn Helen. There are numerous memorial stones of Early Christian affinity dating from the post-Roman period. The post-Roman hillfort of Dinas Emrys also dates to this time. Churches were introduced to the region in the 5th and 6th centuries. Llywelyn the Great and Llywelyn ap Gruffudd had various stone castles constructed to protect their borders and trade routes. Edward I built several castles around the margins including those at Harlech and Conwy for military and administrative reasons. Most are now protected within a World Heritage Site. Some of Snowdonia's many stone walls date back to this period too. The 18th century saw the start of industrial exploitation of the area's resources, assisted by the appearance in the late part of the century of turnpike trusts making it more accessible. The engineer Thomas Telford left a legacy of road and railway construction in and around Snowdonia. A new harbour at Porthmadog linked to slate quarries at Ffestiniog via a narrow gauge railway. At its peak in the 19th century the slate industry employed around 12,000 men. A further 1000 were employed in stone quarrying at Graiglwyd and Penmaenmawr. Mining for copper, iron and gold was undertaken during the 18th and 19th centuries, leaving a legacy of mine and mill ruins today. Ruins of the gold industry are found at Cefn Coch on the Dolmelynllyn estate. Natural history The park's entire coastline is a Special Area of Conservation, which runs from the Llŷn Peninsula down the mid-Wales coast, the latter containing valuable sand dune systems. The park's natural forests are of the mixed deciduous type, the commonest tree being the Welsh oak. Birch, ash, mountain-ash and hazel are also common. The park also contains some large (planted) coniferous forested areas such as Gwydir Forest near Betws-y-Coed, although some areas, once harvested, are now increasingly being allowed to regrow naturally. Flora Northern Snowdonia is the only place in Britain where the Snowdon lily (Gagea serotina), an arctic–alpine plant, is found and the only place in the world where the Snowdonia hawkweed Hieracium snowdoniense grows. One of the major problems facing the park in recent years has been the growth of Rhododendron ponticum. This fast-growing invasive species has a tendency to