They were typically built in the period up to early modern times as part of a city or town wall or castle.
These towers were built in pre-medieval and medieval times to guard access to the bridge and to enable the charging of tolls on important roads crossing rivers, usually near towns and cities.
As a result, it was important from a defensive perspective that the bridges did not allow attacking enemies to break in.
In addition to their genuine protective and defensive functions they also played a symbolic and architectural role.
Often these towers were the first public buildings that the travellers saw when approaching the city.