We visited the Upper St Michaels cave. It was on a trip with the taxi drivers (that you can pick up at Trafalgar Cemetary). You pay one price which includes the Rock, the monkeys (who all seem to know the drivers and do as they are told for them), caves and the Great Siege Tunnels. The caves are very interesting, huge stalagmites and stalagtites and an auditorium where they hold concerts.
The Great Siege Tunnels is more or less an outdoor museum with waxed models and information on plaques explaining how people and the soldiers of the time lived.

Don't get St Michael's cave and Lower St. Michael's cave confused! We visited St Michael's cave as part of a minibus tour of the upper rock. The leaflet from the tourist info office mentioned that a second cave had been found beneath that, and was called Lower St Michael's cave; according to the tourist info office, Lower St Michael's cave requires "some" rope climbing. This made it sound as if it was mostly leisurely walking with the occasional rope hand-rail! Do not believe a work of it! Lower St Michael's cave was a most terrifying experience! To move between the interconnected chambers in the rock you have to climb down rock faces that are 8 to 20 ft drops. There are few, if any, footholds. Gravity pulls you down and you hold a rope to stop you falling too fast! It is not the "occasional" rope-work that the brochure describes; there are ropes in every chamber. Eventually you arrive at an underground lake. At last, you think, you have finished. No. You now have to walk across the water, standing on a ledge at the edge of the pool. The problem is that halfway round the lake the ledge disappears. How no-one got killed or injured amazes me! Only go if you enjoy rock climbing and wear strong boots. We wore trainers and they simply were not good enough. You will not need thick clothing because you sweat so much from exertion, or is that fear?