Bridges can be built over rivers, in order to allow citizens across. Bridges don't seem to have a limit to their length so long as it starts and ends on land. Bridges cannot be built going into a mountain.
It is the third item in the Roads and Bridges category, accessed via the fourth button on the interface bar. Hotkey: F4,3
- Building Cost: 4 Logs and 1 Stone for each unit of length.
- Building Time: 3 Labor for each unit of length.