Windsor Castle

• The lower ward of the Windsor Castle.Brand X Pictures/Brand X Pictures/Getty Images
  
  William the Conquerer built the Windsor Castle in the years following the Norman conquest of England in 1066. The castle was originally built on a raised earthen motte and was fortified with wooden palisades. A central tower and an enclosed courtyard were also a part of its design. Henry I expanded the size of the castle and replaced the wooden palisades with Bagshot heath stone. Additional fortifications were added in 1175 by Henry II.
  
  

Tower of London

• The Tower of London is one of England's most popular tourist destinations.Hemera Technologies/AbleStock.com/Getty Images
  
  The foundations of the Tower of London were laid in 1078. This historic castle was initially built as a simple timber and stone enclosure on the north bank of the Thames River. Over time, a residential palace and a fortress were added. The Tower of London is often referred to as the White Tower. The White Tower is technically the name of the castle's central keep. This name was given to the main tower after William the Conqueror ordered the structure to be built out of French-imported Caen stone.
  
  

Heidelberg Castle

• The Heidelberg Castle was built on a hill above the city.Alexander Hassenstein/Digital Vision/Getty Images
  
  The Heidelberg Castle is one of Germany's most grandiose historical landmarks. The castle was constructed atop the Königstuhl hillside in the 13th century. Large slabs of natural rock and hardened dirt materials were used for the castle's foundation. The exterior of the castle was built from red sandstone. During the following 400 years, other stone structures including ramparts, outbuildings and palaces were added to accompany the original royal residence. The castle was built in the Gothic and Renaissance styles. In the 17th and 18th centuries, the castle was repeatedly damaged by lightning and picked apart by townspeople who used the castle stones to build new houses in the city.
  
  

Edinburgh Castle

• The Edinburgh Castle was built on top of volcanic rock.Medioimages/Photodisc/Photodisc/Getty Images
  
  The Edinburgh Castle was built late in the 11th century and remains today as one of Scotland's premiere tourist destinations. The castle was built atop an extinct basalt-rock volcano 120 meters above sea level. Molten rock was used to build the exterior of the castle. Iron was used to fortify the Witches' Well. The castle's location gave it a natural defense system, making the castle gates only accessible from the east. The first building erected at the castle was a tiny chapel and it was built on the highest point of the hill. Although the stone castle has seen its fair share of sieges and occupations, various restoration projects over the centuries have allowed it to endure.