The sea has borne immigrants, sometimes, doubtless, violent, for millennia, and, with the rise of the nation state, has grown the need to defend against invasion, so a wide variety of structures and installations has been built. It has been argued that Iron Age brochs are primarily defensive strongholds (MacKie 1974), if not primarily in response to potential over-sea invasion. Castles and structures constructed at times of threat, for example against Napoleon and during the two World Wars, are, often, still obvious in the landscape. Guy (1992-9a, b) has recorded many of the defensive systems of the 20th century, and Barclay (2005) has published a study of the Cowie Line, which although extending inland, was part of the defensive system constructed to slow down the enemy should there be a seaborne invasion of the north-east coast of Scotland. This study has shown how the World War II defensive network should be seen as a whole, with sites related to each other despite wide geographic separation. It also demonstrates how the influence of the coast can extend far inland, with one end of the Cowie Line being over 20km from the sea.
Much of the RCAHMS ongoing work is in co-operation with Defence Estates/Ministry of Defence whereby surveys include active airfields in Scotland and other former military sites that are to be relinquished. Most recently, Machrihanish was photographically surveyed and other major surveys have been the Orkney-Scapa Flow Survey 1995-2000, Inchkeith and other Forth Islands and many other smaller former military sites throughout Scotland. The Inchkeith survey included several elements of pre-1900 military installations.
In addition John Guy completed a series of rapid survey reports based on the former Regions for Historic Scotland. The survey concentrated mainly on the coastline and the data has been entered into the RCAHMS database. Secret Scotland (http://secretscotland.wordpress.com/) and other websites provide an ongoing window on the discovery of WW I and WW II sites. There have been several local studies of military sites, available either on-line or in local libraries. Some copies have been supplied to RCAHMS and the location data for these projects will be entered into their database.