Malta has a large number of fortifications from different periods of history, but mostly from the 16th to 20th centuries. In 2003 a set was issued depicting various examples of military architecture of the Maltese Islands. It was designed by Stephen C. Spiteri, one of the main experts on Maltese fortifications.
Images from the MaltaPost online stamp catalogue:
1c Fort St Elmo, Valletta - This fort began to be built in 1552, and it is best-known for its role in the Great Siege of 1565. It was besieged and heavily bombarded by the Ottomans, and it held out for 28 days before falling on 23 June 1565, with most of the garrison being massacred. The long attack on the fort bought time for defenders in other forts, and the Ottomans were eventually defeated after the arrival of a relief force in September 1565. The fort was rebuilt after the siege and it was incorporated into the walls of Valletta. It was heavily altered in later centuries, and it remained in use until 1972, seeing use once again in World War II as well. It now houses the National War Museum.
4c Rinella Battery, Kalkara - This battery was built by the British between 1878 and 1886 to house a single 100-ton gun. Intended to counter the Italian naval threat, the gun was never put into use. The fort and its gun are still intact and today it is a museum.
11c Fort St Angelo, Birgu - This was originally a castle, and no one knows when it was built, although it definitely existed by the 13th century. It became the headquarters of the Knights Hospitaller in 1530, and it withstood the aforementioned Great Siege of 1565. The present layout of the fort is from the late 17th century when it was rebuilt. It remained in use by the British until 1979, and today it is open to the public as a museum.
16c Reserve Post R15, Naxxar - This is one of many pillboxes built around the Maltese Islands in the years immediately before and during World War II. These concrete pillboxes, which included machine gun posts, were meant as defensive positions in the case of an Italian or German invasion.
44c Fort Tigné, Sliema - This was the last fort ever built by the Knights Hospitaller, being constructed between 1793 and 1795. It saw action during the French invasion of Malta in 1798, being heavily bombarded in the process. The fort remained in use, with heavy alterations, by the British until 1979. The area around the fort has now been heavily built up with apartments and a shopping mall, but the fort itself is still in good condition although it's not open to the public.