Sections of the medieval walls of Mdina were rediscovered by Dr. Stephen C. Spiteri and Mario Farrugia while they were surveying the bastions in 2002. The first plans to undertake a major restoration of Mdina's walls were made in 2006, as part of a project that also included restoration of the fortifications of Valletta, Birgu and the Cittadella. In the case of Mdina, the main focus was to consolidate the terrain, since the city is built on a blue clay plateau which is prone to subsidence. The multimillion-euro restoration was partially financed by the European Regional Development Fund. Works began in early 2008 by the Restoration Unit. The process of restoration was documented by Dr. Spiteri. In 2011, steel rods were inserted into D'Homedes Bastion in order to prevent the walls from slipping down the clay slopes. The ditch was inaugurated as a public garden in March 2013. All ERDF-financed work was completed in late 2013, but the Restoration Directorate later restored other parts of the fortifications, including the northern walls, St. Peter Bastion and the area around Greeks Gate. The project was fully completed in early 2016.. How many years did the work on the restoration of Mdina's walls take to become fully completed?