Sea fortress

Coastal defence (or defense) and coastal fortification are measures taken to provide protection against military attack at or near a coastline (or other shoreline), for example, fortification and coastal artillery. Because an invading enemy normally requires a port or harbour to sustain operations, such defences are usually concentrated around such facilities, or places where such facilities could be constructed.

In littoral warfare, coastal defence counteracts naval offence, such as naval artillery, naval infantry (marines), or both.

Rather than the beach assault of modern amphibious operations seaborne assaults of the classical and medieval age more often took the form of raiders sailing up river and landing well inland of the coast. Prior to the invention of naval artillery that could sink hostile ships, the most that coastal defence could do was act as an early warning system, that could alert local naval or ground forces of the impending attack. For example, in the late Roman period the Saxon Shore was a system of forts at the mouths of navigable rivers, and watch towers along the coast of Britannia and Gaul.

Later in Anglo-Saxon Wessex protection against Viking raiders took the form of coast watchers whose duty was to alert the local militia; the navy, which would attempt to intercept the raider's ships or failing that to destroy them after they had beached, against smaller raiding forces the threat of losing their ships, and their way home with their loot was often enough to force them to curtail their attack; and a system of fortified towns, these burghs were positioned at choke points along navigable rivers to prevent raiders from sailing inland.

...
Wikipedia