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Caernarfon is situated on the southern bank of the Menai Strait facing the Isle of Anglesey. It is situated south-west of Bangor, north of Porthmadog and approximately west of Llanberis and Snowdonia National Park. The mouth of the River Seiont is in the town, creating a natural harbour where it flows into the Menai Strait. Caernarfon Castle stands at the mouth of the river. The A487 passes directly through Caernarfon, with Bangor to the north and Porthmadog to the south.