"The word "megalithic" describes structures made of such large stones, utilizing an interlocking system without the use of mortar or concrete, as well as representing periods of prehistory characterised by such constructions." Source: -Wikipedia.
Recent archaelogical expeditions have uncovered megalithic structures in Batanes. These fortresses are called "Idjangs". But these discoveries are still full of mysteries as to what use those structures are for.
[url]http://images.liveinthephilippines.com/content/wp-content/uploads/2010/06/Artist-Sketch-Idjang-300x200.jpg[/url]