Costa Head
Costa Head is a prominent headland
A headland is a point of land, usually high and often with a sheer drop, that extends out into a body of water.Headland can also refer to:*Headlands and bays*headLand, an Australian television series...

 on Eynhallow Sound
Eynhallow Sound
Eynhallow Sound is a seaway lying between Mainland Orkney and the island of Rousay in the Orkney Islands, Scotland. The tidal indraught is "scarcely felt beyond a line joining Costa Head and the Reef of Quendale". An Iron Age broch, Gurness, has a strategic outlook over the Eynhallow Sound....

 on the northwestern coast of the Orkney Mainland, Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

. The tidal indraught of Eynhallow Sound is "scarcely felt beyond a line joining Costa Head and the Reef of Quendale". To the east is the Point of Hellia
Point of Hellia
The Point of Hellia is a headland on the northwest coast of the Orkney Mainland, Scotland. This landform extends into the southern part of Eynhallow Sound, a seaway of the North Sea.Gurness, an Iron Age broch promontory fort, is situated on the Point of Hellia...

 on which is located the Broch
A broch is an Iron Age drystone hollow-walled structure of a type found only in Scotland. Brochs include some of the most sophisticated examples of drystone architecture ever created, and belong to the classification "complex Atlantic Roundhouse" devised by Scottish archaeologists in the 1980s....

 of Gurness, an Iron Age
Iron Age
The Iron Age is the archaeological period generally occurring after the Bronze Age, marked by the prevalent use of iron. The early period of the age is characterized by the widespread use of iron or steel. The adoption of such material coincided with other changes in society, including differing...

 promontory fort
Promontory fort
A promontory fort is a defensive structure located above a steep cliff, often only connected to the mainland by a small neck of land, thus utilizing the topography to reduce the ramparts needed. Although their dating is problematic, most seem to date to the Iron Age...

. Gurness' drystone construction of the "round tower
Round tower
Round tower may refer to:Types of tower:* Irish round tower, a type of early mediaeval stone tower* Broch, a type of Iron Age drystone structure found in Scotland* Round-tower church, a type of church found mainly in England...

 fort is flanked by a number of ancillary structures and impressive concentric ditch and rampart
Defensive wall
A defensive wall is a fortification used to protect a city or settlement from potential aggressors. In ancient to modern times, they were used to enclose settlements...

 outer defences; moreover, the rocky shoreline cliff
In geography and geology, a cliff is a significant vertical, or near vertical, rock exposure. Cliffs are formed as erosion landforms due to the processes of erosion and weathering that produce them. Cliffs are common on coasts, in mountainous areas, escarpments and along rivers. Cliffs are usually...

s posed a formidable approach for marine invaders."
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