Golkonda Fort, Hyderabad, Andhra Pradesh is situated on a 300 foot Granite hill and was initially constructed in 10 CE. It was reconstructed in its present form by the King Ibrahim of Qutub Shahi dynasty. The acoustic system was designed in a way that a clap on the bottom of the hill will be audible on the top (over 1 KM in distance). Golkonda is also famous for its diamond mines – the famous Koh-i-noor, Darya-i-noor & Hope diamonds were excavated from here. More pics here.