The only respect in which the bridge of Xerxes differed from constructions with which the Persians were thoroughly familiar, was in its superior solidity and strength.

The shore-cables were of unusual size and weight, and apparently of unusual materials; the formation of a double line--of two bridges, in fact, instead of one--was almost without a parallel; and the completion of the work by laying on the ordinary plank-bridge a solid causeway composed of earth and brushwood, with a high bulwark on either side, was probably, if not unprecedented, at any rate very uncommon.

Boat-bridges were usually, as they are even now in the East, somewhat rickety constructions, which animals unaccustomed to them could with difficulty be induced to cross.
The bridge of Xerxes was a high-road, as AEschylus calls it along, which men, horses, and vehicles might pass with as much comfort and facility as they could move on shore.
The utility of such a work is evident.

Without it Xerxes must have been reduced to the necessity of embarking in ships, conveying across the strait, and disembarking, not only his entire host, but all its stores, tents, baggage, horses, camels, and sumpter-beasts.

If the numbers of his army approached even the lowest estimate that has been formed of them, it is not too much to say that many weeks must have been spent in this operation.