After Edward I of England (Longshanks, to those of you who have seen Braveheart) conquered Wales in 1284, he extracted cooperation from the Welsh by promising them that their next ruler would be “a prince born in Wales who did not speak a word of English.” When Edward’s son (later to become Edward II of England) was born several months later in Caernarfon Castle, Wales, he was presented to the Welsh people as the first Prince of Wales. The Welsh people had no choice but to accept him as their ruler; the baby didn’t speak a word of English.
Since then, the heir apparent to the reigning monarch of England has been titled the Prince of Wales. Prince Charles, the current Prince of Wales, was officially invested in Caernarfon Castle in 1969.
Caernarfon was one of 17 castles that Edward built in Wales after 1284 to assimilate the English into Wales. He felt the need, however, to build heavily fortified settlements for the English so that the Welsh would not have access.