B1-series battle droid traversing a plasma bridge
A plasma bridge served as a technological means of controlling passage to a particular zone. Functionally, it was a span of projected energy, strong enough to bear the load of individuals, repulsorlift vehicles, and terrestrial transports. To instantly block entry, the bridge could be turned off, causing anyone in the span to fall.
The concept of a plasma bridge is a futuristic adaptation of the drawbridges commonly seen protecting medieval fortresses.