Visas and money. Wales is part of the United Kingdom, so UK entry rules apply, and the currency is the pound sterling. Cards are accepted almost everywhere, including at most car parks and village shops, but carry a little cash for honesty boxes and the odd rural café.
Language. Wales is officially bilingual, and every road sign, station and public notice is in Welsh and English. Everyone speaks English, but Welsh is the first language for many people in the north and west. A diolch (thank you) goes down well. Place names make more sense once you know a few words: llyn is lake, aber a river mouth, cwm a valley, pen a summit and eglwys a church.
Heritage sites. Most castles and ruined abbeys are run by Cadw, the Welsh government's heritage service. If you are planning to see several, look at the Cadw Explorer Pass or membership. The National Trust runs many of the country houses and much of the coast.
Safety. Wales is a safe place to travel. The real risks are outdoors: fast-changing weather on the mountains, tides that cut off beaches and causeways, and strong currents. Check the forecast before going high, and the tide times before walking out to an island or a sea cave.