This decent museum has a fine diorama of the site, providing an excellent overview and explaining Sigiriya's cultural importance beyond the obvious natural beauty. The theory that Sigiriya was a Buddhist monastery is given here, although the established position that it was a palace or fortress prevails. Trade routes are explained, showing Sigiriya's connections with the Gulf, China, India and the Roman Empire.
Look for the wonderful black-and-white photos of British archaeologist HCP Bell and his somewhat reluctant family exploring the site soon after its rediscovery. The museum is near the main ticket booth. No photographs are permitted inside.