With Strongholds, the mediocre defensive capabilities of Celts become much stronger. Unfortunately, most of the troops who dare besiege a Castle are already either virtually immune to arrow fire, or simply outrange the Castle.
While the Celts were not the only ones to establish castles in Europe as a means of fortifications, during the Middle Ages, they constructed countless of castles and fortifications across the rugged highlands of Scotland. The combination of difficult terrain and fortifications protected the land from the British incursions into their territory until Edward I's conquest. The legacy of these strongholds included Edinburgh and Caerlaverock which still stands today as the center of Scottish authority.