A large part of the Snowdonian Range lay in the centre and south-east of the former county, which included Snowdon itself, the highest mountain in Wales at 1,085 m (3,560 ft).

The south-west of the county was formed by the Llŷn peninsula, with Bardsey Island lying off its western end.

[2] The counties included the city of Bangor and the towns and villages of Betws-y-Coed, Caernarfon, Conwy, Llandudno, Porthmadog and Pwllheli.

The county was originally created under the terms of the Statute of Rhuddlan in 1284 following Edward I of England's conquest of the Principality of Wales and included the cantrefi of: Llŷn, Arfon, Arllechwedd and the commote of Eifionydd (the northern portion of Dunoding).

[3] The county was divided into ten hundreds based on the existing Welsh commotes: Cymydmaen (anglicised as Commitmaen), Creuddyn, Dinllaen, Eifionydd (Evionydd), Cafflogion (Gaflogion), Llechwedd Isaf (...Isav), Llechwedd Uchaf (...Uchav), Nant Conwy (Nant-Conway), Is Gwyrfai (Isgorvai) and Uwch Gwyrfai (Uchgorvai).

[14] renamed Porthmadog UD 1915[22] The civil parish of Llysfaen was a detached exclave of the county.

It was largely split between the three districts of Aberconwy, Arfon and Dwyfor of Gwynedd (along with Merionethshire and Anglesey).

Caernarvonshire County Council received a grant of armorial bearings from the College of Arms in 1949.

According to the poet Michael Drayton, the eagles formed the device on the banner of the Caernarvonshire soldiers at the Battle of Agincourt.

The pattern of three gold eagles on a green background is a design with a long association with the county, having reputedly been flown by Caernarfonshire soldiers at the Battle of Agincourt in 1415.