The impressive building that housed the Customs’ House of the island during the period of economic growth and prosperity of Syros is situated at the port. The building has been designed and realized by the Greek architect Alexandros Georgantas in the years 1859-1861. It is a two-storey building made of marble with isodomic masonry and many features also made of marble.

The upper floor has bi-foiled and tri-foiled openings which give to the building an imposing appearance, whereas the façade ends look like towers. A series of columns ending up to arches form an oblong gallery. The roof of the building has wide eaves with dense decorative features which give an excellent impression. The whole style of the building reminds the buildings designed by the eminent Greek architect Stamatios Cleanthis to serve as residencies of the well known Duchesse of Plaisance; Georgantas was the supervisor of the construction of these buildings.

During the bombing of the island in the World War II a considerable part of the Customs’ House collapsed. The restoration work started immediately after the end of the war with full respect to the initial design of the building. By this period of time it has been constructed the waterfront for the mooring of ships which are subject to customs inspection. The square in front of the Customs House is also an after war construction. For the construction of the square it had been necessary to demolish the old charcoal storehouses and embank the area. Since 1974 the square is decorated by the Monument of the Unknown Seaman, a work of the well known Greek sculptor Ioannis Pappas. The sculptor had been sponsored by the family of Goulandris, the well known Greek ship owners. Another interesting building at this square is the building of the Nautical Association of Syros; the association was established in 1933 and since 1969 it is housed in the nice building designed by the well known Greek architect Michalis Tsironis. From the square the visitor can see in the distance the small islet of Gaidaros or Didymo, with the tall stone lighthouse.