Slavic pirates who routinely menaced Venetian shipping lanes in the Adriatic. A series of fortifications was consequently erected during the reign of Doge Pietro Tribuno () to protect Venice from invasion by sea. These fortifications included a wall that started at the rivulus de Castello (Rio del Palazzo), just east of the Doge's Castle, and eventually extended along the waterfront to the area occupied by the early Church of Santa Maria Iubanico. However, the exact location of the wall has not been determined nor is its duration beyond the moment of crisis indisputable. Integral to this defensive network, an iron harbour chain that could be pulled taut across the Grand Canal to impede navigation and block access to the centre of the city was installed at the height of San Gregorio. In addition, a massive watchtower was built in Saint Mark's Square. Probably begun during the reign of Tribuno, it was also intended to serve as a point of reference to guide Venetian ships safely into the harbour, which at that time occupied a substantial part of the area corresponding to the present-day piazzetta. Construction Tower The defensive system begun under Pietro Tribuno was likely provisional, and construction may have been limited to the reinforcement of pre-existing structures. Medieval chronicles suggest that the laying of the foundation for the tower continued during the reigns of his immediate successors, Orso II Participazio () and Pietro II Candiano (). Delays were likely due to the difficulty in developing suitable construction techniques as well as locating and importing building materials. Some of the early bricks dated from the late Roman Empire and were salvaged from ruins on the mainland. For the foundation, alder piles, roughly in length and in diameter, were driven into a dense layer of clay located around below the surface. The piles were topped with two layers of oak planking on which multiple layers of stone were laid. Fabrication of the actual tower seems to have begun during the brief reign of Pietro Participazio () but did not progress far. Political strife during the ensuing reigns of Pietro III Candiano () and, particularly, Pietro IV Candiano () precluded further work. Under Pietro I Orseolo (), construction resumed, and it advanced considerably during the reign of Tribuno Memmo (). No further additions were made to the tower until the time of Domenico Selvo (), an indication that it had reached a