I recently stumbled on a cache of diagrams detailing various Jamaican caves.
Here's Volcano Hole, which looks quite formidable:
The entrance is a vertical shaft that drops 100 meters. This sort of drop serves as a pretty simple barrier between the surface world and the subterranean one, and presents an interesting access challenge. Going down is easier than going up.
Here's Bottom pasture Cave 2:
This looks like a lair. You've got a capacious and obvious entrance, and a second, smaller entrance (exit?) that is easy to conceal. The rear of the cave is extremely confined, narrowing down to sub-one-meter dimensions and going on for a long way. Crawling around back there would make one very vulnerable, and it ends in water.
Here's the St. Clair cave:
This one is topographically interesting, but also illustrates two hazards worth mentioning (and incorporating in your imaginary cave systems) - animal-borne pathogens like histoplasmosis and dangerous gas mixes. The air down there might not be fresh. It might kill you, and if it doesn't, the bat funk eventually might.
One last example, the Vaughensfield cave:
The plan of this cave is very typical - it isn't a crenelated mess of twisting passages - it is a straight line gradually descending into the earth. The occasional vertical shaft pokes through into the forest above, providing some murky light (and perhaps a point of ambush or escape for small, limber creatures that live in the cave).