Stirling Castle, located in Stirling, is one of the largest and most important castles, both historically and architecturally, in Scotland. The castle sits atop Castle Hill, a volcanic crag, which forms part of the Stirling Sill geological formation. It is surrounded on three sides by steep cliffs, giving it a strong defensive position. Its strategic location, guarding what was, until the 1930s, the farthest downstream crossing of the River Forth, has made it an important fortification from the earliest times. Most of the principal buildings of the castle date from the fifteenth and sixteenth centuries. A few structures of the fourteenth century remain, while the outer defences fronting the town date from the early eighteenth century. Several Scottish Kings and Queens have been crowned at Stirling, including Mary, Queen of Scots, in 1543. There have been at least eight sieges of Stirling Castle, including several during the Wars of Scottish Independence, with the last being in 1746, when Bonnie Prince Charlie unsuccessfully tried to take the castle.