A powerful defensive structure built specifically to house heavy guns in multiple tiers. The castle walls were built of stone, sometimes with earthen outworks. They were usually constructed to protect a harbour entrance or anchorage from attack by sea. Accommodation inside the castle was usually limited to that needed for the garrison and to store munitions. One of the earliest examples is Dartmouth Castle, Devon, built in ad 1481; the majority were constructed in the 16th century ad.