They are classified as igneous rocks by how they are formed.(melted and then cooled) So the earths crust is subducted into the asthenosphere where it is mostly magma and melts. Then the magma rises towards the surface in currents and cools along the ways. The deeper they start forming the lager the crystals.

High ground is always an advantage over the enemy in military tactics but also to prevent from being over taken or surrounded the union had to defend crucial places like little round top.