Palladian is a style of architecture which was popular in the 18th Century. It attempts to recreate the style and proportions of the buildings of ancient Rome.
Most people think there are only 4 Palladian Bridges in the world. In fact there are 5. Catherine the Great commissioned one in St Petersburg.
The one at Prior Park was designed by the 18th landscape gardener Capability Brown for local businessman Ralph Allen whom, from humble beginings, made a fortune (not always honestly) from developing the national postal services and later mining Bath stone. Prior Park was built primarily as a marketing exercise to show what could be achieved with his product.