The two bridges across the Rhine river are considered among the greatest military engineering achievements in the ancient world.
Caesar's Bridge across the Rhine, the first two bridges to cross the Rhine River on record, were built by Julius Caesar and his legionaries during the Gallic War in 55 BC and 53 BC. Strategically successful, they are also considered masterpieces of military engineering.1
What is not clear to me is why the tribes across the Rhine, who believed they could stop any crossing legions in boats by harrying them during their crossing, didn't employ the same tactics while the bridge was being built.