Answer by Ratnakar Sadasyula:
The Brihadeeswara Temple in Tanjore, India is an awesome feat of engineering. The top of the temple contains a single block of stone, also called as the kalasam, weighing around 50 tons. It was believed that a ramp was constructed to transport the block to the top, using elephants. It is also astounding to note that the temple is more than 1000 years old.
The Kailashnatha Temple in Ellora, Maharashtra, has to be one of the greatest feats of engineering ever. Carved out of a single stone, the temple is unique for it’s top down construction feat. The sculptors, engineers started at the top of the rock and then carved their way straight down, making the temple out of the rock.
The Sun Temple in Konark, representing the Sun drawn in the form of a chariot, with horses, constructed in such a way, that the Sun rays fall right on the sanctum sanctorum at sunrise.
The Daulatabad Fort now mostly in ruins, but had one of the most elaborate security systems, with cannons, booby traps, dead turns, mazes.
The Golkonda Fort, near Hyderabad, with a highly advanced accoustics system, where you could clap at the entrance of the fort, and it would be heard all the way up to the king.
The Gol Gumbaz at Bijapur, famed for it’s Whispering Gallery, again another great feat of accoustical engineering.
The Konkan Railway read it up here http://en.wikipedia.org/wiki/Konkan_Railway