Siege causeways

[D B Campbell]

The oldest reference I found yet is the siege of Motya in 397 BC, the next one the famous one of Tyre in 332 BC.

Of course, there was already a causeway at Motya, which the inhabitants attempted to sabotage. All Dionysius did was to repair it. And at Tyre, they now reckon that the water was fairly shallow for Alexander's "mole".