Click "play" to start the tour. Then either click the circles on the floor, or grab and drag the image, or use arrow keys to move and look around. (A mousewheel also acts like an up/down arrow key.)
Click "Change view" (lower left) to see a map. "Fullscreen" (lower right) is self-explanatory. The "Play" button inside the tour switches to slideshow display mode.
Interesting to know: each pale circle on the floor was a camera position! There are a lot of them.
A little game: try to spot where the camera took a self-portrait. ;-)