A volcanic plug is formed when pressure causes magma to build up in a vent which can then solidify. This rock is harder than the surrounding rock which can get eroded away while the magma core stays intact. I know that you are thinking that this mass of hardened mass of magma seems like a great place to build a fortress. Trotsky castle (Czech Republic) which was once home to Good King Wenceslaus, Edinburgh Castle and Sigiriya Rock Fortress in Sri Lanka are all built on volcanic plugs. Sigiriya as well as being on part of an old volcano is home to beautiful frescos, it was once used as a Buddhist monastery and has some of the oldest examples of landscape garden and urban planning in the world. But I think the most exciting thing about Sigiriya is that it has a mirrored wall so the king could look at himself while he walked, the mirror was made of well polished stone and has inscriptions from the 8th century. It is believed that the complex was built by King Kashyapa as a retreat and leisure palace (I wonder if people will think of Butlins as such a thing of wonder in the future) in around 480 AD however its use as a monestary dates from around 5th Century BC.
The entrance has lions feet!
It's basically my new favourite place