|Length:||19.7 miles (31.7 km)|
|Meets:||A487, B4410, A498, B4418|
|Route outline (key)|
The road starts at a junction with the A487 at Penrhyndeudraeth, and heads northwards towards the mountains. Passing the Ffestiniog Railway station of Penrhyn and a gated level crossing, the road heads out into the plains. A few sharp corners and narrowings of the road take us to a bridge under the Welsh Highland Railway, where the road enters the Aberglaslyn Pass and across a bridge over the River Glaslyn before a T-junction on the A498. There is a multiplex with the A498 along the Aberglaslyn Pass into Beddgelert where the A498 TOTSOs right in the village centre after a bridge, with the A4085 regaining its number by bearing left.
The road now follows the Nant Colwyn along the south edge of Snowdon and continues to shadow the Welsh Highland Railway by crossing it over numerous bridges. After going through Rhyd Ddu the road narrows for a short distance to go alongside Llyn Cwellyn. After leaving the Snowdonia National Park the railway line is also left just before Waunfawr, as it takes a different route to Caernarfon. The road then continues for another couple of miles, crossing the River Seiont and entering Caernarfon from the south east. The road ends at a gyratory system to the east of the town centre back on the A487.
What is now the A4085 was originally the mainline of the A487. However, after following the route it is obvious that the trunk road along the west coast of Wales had to follow an easier route. The B4410 to the west of the mountains was upgraded to Class I status in 1935, becoming the A4085, and that is the way the trunk road went. In the 1960s the numbering was changed so that the more important road number followed the trunk road and so the A487 and A4085 swapped numbers, giving the road its current route.