Complex sentence: The Ellot were responsible for supplying the fort/castle with soldiers, needed, first for the northern outpost of Danish Northumbria a fort, then after the Declaration of Aberoath, for Scotland, but the same fort now called The Hermitage.
Split into simpler sentences: The Ellot were responsible for supplying the fort/castle with soldiers, needed. This was first for the northern outpost of Danish Northumbria a fort, then after the Declaration of Aberoath, for Scotland, but the same fort now called The Hermitage.