Between the Giant’s Causeway and the busy harbor town of Portrush, the coastline is dominated by the hulking skeletal outline of what must have once been a glorious castle. This was the main fort of the Irish MacDonnells, chiefs of Antrim. From the 14th to the 17th century, it was the largest and most sophisticated castle in the North, with a series of fortifications built on rocky outcrops extending into the sea. In 1639, part of the castle fell into the sea, taking some of the servants with it; soon after that, it was allowed to fall into a beautiful ruin. The 17th-century courtyard survives, including a few buildings. The site incorporates two of the original Norman towers dating from 1305. One enticing footnote: A recent archaeological dig here uncovered the remains of a town that was previously thought completely destroyed during a rebellion in 1641. Only a very tiny fraction of what is now thought to exist has so far been excavated.