Although the Instructions to the right say hold "Shift+Spacebar+Mouse – Zoom in metal." The first screen says hold Shift or Space and indeed either works. And yes the last level is vastly more difficult than the preceeding ones. Excellent game, could do with more levels between 19 and 20 in difficulty.
 "Metal Power"){kind=icon}
 "Threesome"){kind=icon}
 "Shields of Gemland"){kind=icon}
 "Dragon Age: Journeys"){kind=icon}