The Qutb Minar is really impressive in person. It is unfathomable how something that big (72.5 meters) and made of stone could be built in 1368 (and started in 1193!)

The Iron Pillar (at left) is also genuinely amazing. Manufactured and installed circa 4th century A.C.E. The pillar is constructed of such pure iron that it is visually completely rust free. The pillar has often been the source of much discussion amongst scientists.