Defense line
Defense line or fortification line is a geographically-recognizable line of troops and armament, fortified and set up to protect a high-value location or defend territory during an armed conflict. A defense line may be based on natural difficult terrain features, such as rivers or marshes, mountain ranges, or coastlines; temporary field fortification works such as trenches; and more permanent fortifications such as fortresses and bunkers. List of defense lines. Some notable defense lines include: