This simple stone archway may have been constructed largely for practical reasons - the buildings on either side of it may have threatened collapse, and the arch might have been built to provide mutual support. Certainly, this is a common reason for the construction of such an arch.
Whatever the initial purpose of the arch may have been, it serves as an entryway to the town, separating its waterfront from the town itself. Observe that the street (which, in this case, is actually a flight of stairs) narrows just beyond the arch, and that the street is in fact very narrow, probably two meters at most.
Although not directly related to the gate, there are several other interesting features shown here. Note first of all that the floors of the center building are corbelled - the building actually becomes larger as it goes up. This serves the function of keeping water off the lower walls and greatly reduces maintenance costs. The roof overhangs the building by a wide margin, further protecting the walls from water damage. The overhang above the street also provides a considerable degree of shelter to those passing below. Finally, notice how plants have been integrated into the buildings themselves, giving them an organic quality. It is likely that the plants serve a practical purpose as well - they probably help to shelter the building from the scorching sun.
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