A national redoubt or national fortress is an area to which the (remnant) military forces of a nation can be withdrawn if the main battle has been lost or even earlier if defeat is considered inevitable. Typically, a region is chosen with a geography favouring defence, such as a mountainous area or a peninsula, to function as a final holdout to preserve national independence and host an effective resistance movement for the duration of the conflict.

The UNESCO World Heritage Site known as the Defence Line of Amsterdam is a 135-kilometre (84 mi) ring of fortifications around Amsterdam. It has 42 forts that are 10–15 kilometres (6.2–9.3 mi) from the centre and lowlands, which can easily be flooded in time of war. The flooding was designed to give a depth of about 30 centimetres (12 in), too little for boats to cross. Any buildings within 1 kilometre (0.62 mi) of the line had to be made of wood so that they could be burnt and the obstruction removed.