Short answers, pulled from the story.
The Menai Strait is a tidal channel about 25 km long that separates the island of Anglesey from Gwynedd on the Welsh mainland. It runs between Caernarfon Bay to the south-west and Conwy Bay to the north-east, both of which are inlets of the Irish Sea.
The Menai Strait was carved by successive Pleistocene ice sheets that scoured the bedrock from northeast to southwest, creating a series of linear hollows. The deepest of those hollows flooded with seawater around 10,000 BC as global ocean levels rose at the end of the last ice age.
Two bridges cross the Menai Strait. The Menai Suspension Bridge, designed by Thomas Telford and opened in 1826, carries the A5 road and is grade one listed. The Britannia Bridge, designed by Robert Stephenson and opened in 1850, carries the North Wales Coast Line and the A55 road; it was rebuilt between 1970 and 1972 after fire damage.
Differential tides at the two ends of the strait create very strong currents. The most hazardous area is the Swellies, between the two bridges, where near-surface rocks produce overfalls and whirlpools; the training ship HMS Conway was lost there in 1953. Shifting sand banks at Caernarfon bar add further danger at the strait's south-western entrance.
The Menai Strait holds status as a special area of conservation with marine components and is a proposed Marine Nature Reserve, partly because its unusual tidal conditions and low wave heights produce a unique and diverse benthic ecology. Its banks are also home to the Menai Whitebeam, a critically endangered Sorbus species with a global population of about 30 plants, all in this part of North Wales.
A Viking raid on Anglesey in the 12th century, recounted in the Orkneyinga Saga, was the occasion on which Magnus Erlendsson, the future Saint Magnus, refused to fight and instead sang psalms on board his ship. The event was retold in George Mackay Brown's 1973 novel Magnus and in Peter Maxwell Davies's 1977 opera The Martyrdom of St Magnus, whose first part is titled The Battle of Menai Strait.