In point of fact, England is quite heavily castellated, though there were few permanent fortifications built after 1600.

Fortifications have always existed in England. Even before the Romans invaded, the native Britons utilized large hill forts defended by ditches and earthworks. The Romans obviously built considerably more fortifications during their occupation. Most people know about Hadrian's Wall, a chain of fortifications stretching from the Irish Sea to the North Sea, built to contain the raids of the Picts. What most people don't know is that there were also significant works constructed in modern Wales and on the southeast coast of England, for the purpose of defending against Irish and Saxon raiders, respectively.

After the Romans left, Britannia fell into a period of chaos and extreme decentralization, during which little military engineering took place. But, as the various Germanic-speaking territories consolidated into larger and more powerful kingdoms, they began to build again. Offa's Dyke, constructed in the 8th century, was a large earthwork built to defend against the Welsh. But the greatest time of fortress building prior to the Norman Conquest must surely have been during Alfred the Great's reign. To defend against large Viking armies that had already toppled several English kingdoms, Alfred embarked on a large scale project of fortification. The burghal system implemented by Alfred saw large fortified towns (by early medieval standards; think 1,000-2,000 people) formed throughout Wessex, to which the local population could retreat for protection. These were not strictly military forts, and they were defended and repaired by local people who were obligated to do so.

When William of Normandy invaded in 1066, these fortified towns still existed, but there was no comprehensive system of defense as existed on the continent. To secure his control of his new kingdom, William began building castles, and the lords he bestowed land on did so as well, though on a smaller scale. Many were located near, or in, the burghal towns. Others occupied strategic positions: river crossings, frontiers, harbors, etc. These castles were initially built from timber and earth, but beginning in the 12th century, they were gradually rebuilt or replaced by stone castles. Extensive castle building continued until at least the 14th century; the Scottish border became particularly heavily castellated.

The last significant period of fortification occurred under the Tudor monarchs, especially Henry VIII. He built a number of artillery forts on the coast - low, thick-walled structures equipped with heavy guns and intended to withstand cannon fire. These were never really tested, and his successors did not build very much at all. This is because, with the exception of William of Orange's invasion (which was invited by the English), England did not face serious prospect of foreign invasion until WWII. The navy was sufficient to protect the coast. Simultaneously, the centralization of power and the growing strength of the state greatly reduced the threat posed by rebellious lords and land-grabbing rivals.