Fosos

Fosos are man-made ditches or moats that encircle a fortification, city wall, or defensive perimeter. The term comes from Latin fossa, meaning ditch or trench. In fortifications, a foso may be dry or filled with water and is often combined with revetments, walls, or ramparts to hamper assaulting forces and protect the defended works.

Dry fossos are simply excavated trenches that rely on depth, slope, and width to slow attackers, while wet fossos contain standing water and can complicate crossings and bridge assaults. Some designs feature partial water, seasonal flooding, or strategic connections to water sources to sustain the moat.

Functions include delaying and disrupting infantry and armor, preventing undermining of walls, and guiding attackers toward controlled points such as gates or bridges. Moats also act as a drainage feature and contribute to the overall landscape and visibility from the defendable line.

Construction typically involves excavating earth and lining the sides with stone, brick, timber, or earth revetments. The moat is usually accompanied by a bridge, drawbridge, or gatehouse, and by a glacis or earthen slope to enhance defensive advantage. Maintenance is required to keep water levels and structural integrity.

Fosos are attested in many regions and periods, from medieval European fortifications to ancient and pre-modern walls that used moats as multipurpose barriers. In modern times, they survive mainly in preserved castles, historical sites, and as landscape features that evoke historic fortifications.

Related terms include moat, trench, and ditch.