A barbican is a fortified outpost or fortified gateway, such as at an outer defense perimeter of a city or castle, or any tower situated over a gate or bridge which was used for defensive purposes.

A siege is a military blockade of a city, or fortress, with the intent of conquering by attrition, or by well-prepared assault. Siege warfare is a form of constant, low-intensity conflict characterized by one party holding a strong, static, defensive position. Consequently, an opportunity for negotiation between combatants is common, as proximity and fluctuating advantage can encourage diplomacy.