The Metal Monster by Abraham Merritt

Set in the remote Himalayas, the METAL MONSTER follows a group of botanists who discover the seemingly reanimated Darius III of Persia and legions of soldiers. The group is saved by a mysterious woman, Norhala. Seemingly magical, Norhala inhabits a hidden city and controls strange metal automatons capable of joining together and forming colossal monsters, each possessing powerful weapons fully capable of defeating all the armies on Earth.