The word 'castle' originated from the Latin word castellum, which is short for 'castrum' meaning 'a fortified place'. Castles were supposed to be a place where prisoners were detained. Also, knights or lords could conduct entertainment programs there. Originally built for purposes of defense, castles soon became comfortable homes for monarchs. Aesthetics gained importance and the size of a castle began to be linked with the status of its occupants. The Windsor Castle that was built for defense purposes is now the British Queen's residence. Most castles of the early days used the motte-and-bailey design. A circular hill surrounded by a dry ditch and flattened at the top characterized this design. A horseshoe-shaped bailey was attached to the mound. Around the summit of the mound, a palisade of timber was constructed to act as a wall. Inside the palisade was a citadel. It is typical to see a curtain wall surrounding castles of the olden days. A reinforced shield wall was built in addition to the curtain wall. There were many gates for the castle and a drawbridge guarded its main entrance. This made it difficult for intruders to gain easy access to the castle. The parapets that projected over the main gate could be used to drop missiles or boiling liquids on the enemies. A stronghold was the tallest and the strongest building in the interior of a castle. Apart from being the place of utmost safety, it was also a status symbol for the occupants. Since hundreds of years before Christ, that is, since the Neolithic Age, people have been constructing hill forts. Roman forts were among some of the early constructions. They built forts that were generally rectangular and had rounded corners. According to Roman engineer Vitruvias, rounded corners enabled a more efficient use of stone. Moreover, the roundedness increased their capacity to defend siege engines that were meant to break down the fortification walls. Rounded corners also improved the field of fire. Some of the earliest castles were built in the 9th century. Wood, earth, and stone were used in their construction. During the Middle Ages, many Roman constructions were converted into castles. In the 10th century, stone began to replace wood as it would provide better protection. With the emergence of feudalism, the construction of castles received greater impetus. Castles were then the residences of lords, and places of refuge for their followers. In the 11th century, the British Isles were introduced to castles. With his Norman Conquest of England, William the Conqueror came up with the idea of building of castles to protect lands. Mostly, stone was used in the construction of castles. In case of its shortage, they were built using bricks. Brick castles are prominent in the Baltic and Scandinavian regions. The glorious history of castles, their design and engineering, and the interesting stories behind their construction, continue to intrigue historians and tourists alike. It requires human intellect to conceptualize a civil structure, and great skill to bring it into reality. Castles weren't an exception. The building of castles required ditch diggers, masons, carpenters, stonecutters, and efficient engineers. Though medieval machines were used for their construction, castles would often take years to complete. Originally, castles were meant to serve military purposes. Over time, their construction style improved. Today, many of them are tourist attractions.