Penrhyn Bridge, including parapet walls on the approach road to Penrhyn Castle Port

Penrhyn Bridge, which includes the parapet walls on the approach road to Penrhyn Castle, is constructed of coursed rubble and features a single segmental arch made with voussoirs, a keystone, and an arch ring. An iron plaque with raised lettering is positioned above the arch. The parapet appears to have been rebuilt and is topped with an ironwork balustrade. At either end of the bridge, there are advanced piers that have tall concave recesses and bases that sweep out. The roadway is supported on both sides by revetment walls that are slightly swept out.

The parapets extend on the eastern bank towards the entrance of Penrhyn Castle, featuring regularly spaced low piers. On the northern side, the parapet rises over a garden door and curves back to end at a large square pier next to the Customs House. On the southern side, the parapet continues to the gate leading into the adjacent field. At the western end, the wall slopes down on the northern side and rises to join the boundary wall that ascends to Llandegai Road. Historical photographs indicate that the bridge originally had iron gates at the western end.

At the eastern end, below the roadway, there are two segmental arched openings with boarded doors, one of which on the northern side is partly bricked up. These openings were originally used for the LNWR Port Siding and Penrhyn Quarry Railway, which had tracks that led directly to the harbour.