estimation of additional thousands coming from countries outside the EU (mostly from the African continent) that are under illegal alien status and therefore are not accounted for in official population figures. The real percentage of foreign residents is higher, since the Alicante metropolitan area is home to many Northern European retirees who are officially still residents of their own countries. A sizable number of semi-permanent residents are Spanish nationals who officially still live in Madrid, Castilla y León, the Basque country, or other areas of Spain. Transportation Alicante Airport outranks its Valencian counterpart, being among the busiest airports in Spain after Madrid, Barcelona, Palma de Mallorca and Málaga. It is connected with Madrid and Barcelona by frequent Iberia and Vueling flights, and with many Western European cities through carriers such as Ryanair, Easyjet and Jet2.com. There are also regular flights to Algeria and Russia. Alicante railway station is used by Cercanías Murcia/Alicante commuter rail services linking Alicante with suburbs and Murcia. Long-range Renfe trains run frequently to Madrid, Barcelona, and Valencia. In 2013, the Madrid–Levante high-speed rail network was extended to Alicante station, allowing AVE high-speed rail services to link to Madrid via Villena AV, Albacete-Los Llanos and Cuenca-Fernando Zóbel. Alicante Tram connects the city with outlying settlements along Costa Blanca. , electric tram-trains run up to Benidorm, and diesel trains go further to Dénia. The city has regular ferry services to the Balearic Islands and Algeria. The city is strongly fortified, with a spacious harbour. Main sights Amongst the most notable features of the city are the Castle of Santa Bárbara and the port of Alicante. The latter was the subject of bitter controversy in 2006–2007 as residents battled, successfully, to keep it from being changed into an industrial estate. The Santa Bárbara castle is situated on Mount Benacantil, overlooking the city. The tower (La Torreta) at the top, is the oldest part of the castle, while part of the lowest zone and the walls were constructed later in the 18th century. The promenade Explanada de España, lined by palm trees, is paved with 6.5 million marble floor tiles creating a wavy form, and is one of the most lovely promenades in Spain. The Promenade extends from the Port of Alicante to the Gran Vía and ends at the famous statue of Mark Hersch. For the people of Alicante, the promenade is the meeting place for the traditional Spanish