A bridge is a construction linking, at a raised level, two pieces of land -- usually land parted by a river or a chasm. It offers travel across an otherwise difficult or impenetrable barrier, but by its nature acts as a choke on mobility since only so many travellers can use it at once, and often in only one direction at once.
For this reason storytellers were prone to cast Arthurian knights as meeting foes at bridges, where a battle, or a passage of arms had to occur in order to achieve passage. From this it has come to be a standard or trope in recreation wars, where narrow passages must be assaulted/defended over a set period. Understandably, these are referred to as bridge battles.
Period bridges were made of wood, dirt or stone.