|To:||Groeslon roundabout (SH558668)|
|Length:||5.3 miles (8.5 km)|
|Meets:||A4086, A4244, B4547|
|Old route now:||A4244, B4409|
|Route outline (key)|
The B4366 is a largely rural B-road heading east out of Caernarfon. The road was originally unclassified but had gained its number by 1932.
The road starts at a roundabout on the A4086 on the eastern side of Caernarfon and heads northeast out of town along the residential Bethel Road. Open country is reached not long after crossing a roundabout serving a school and leisure centre, then the road continues in the same direction with the hills of Snowdonia visible in the distance.
After crossing another roundabout the road reaches Bethel and skirts the southern edge of the village, with few buildings on the right side of the road. A mile or so later the road ends at a roundabout on the A4244 and B4547.
When originally classified the B4366 continued ahead along what is now the A4244 and then the B4409 into Bethesda to end on the A5. Following road improvements to the central section it was finally upgraded to become the A4244 in the 2000s, with the eastern section presumably being renumbered at this time to avoid a multiplex.