The Kremlin is a fortified complex in the center of Moscow, Russia, made up of more than 15 buildings, 20 towers, and more than 1.5 miles of walls that are up to 21 feet thick.

Since 1991, it's been the official residence of Russia's president, currently President Vladimir Putin.

The complex is protected by an elite military regiment whose members must be able to hear a whisper from 20 feet away.