Yes, Dagobah is primarily a swamp planet, teeming with life, as we all know. But the terrain isn't all "sea level," so to speak; hidden within the thick jungles here and there are ancient plateaus and mesas, created by the planet's natural (and severely slowed) tectonic forces, which are covered by jungle as well, and difficult to find on foot since because of all the growth they are fairly well hidden to the casual eye. One can be literally trudging through the level swamp/bog/jungle and only know that he or she is approaching one of these immense geomorphological structures only by a slight feeling of elevation gain, indicating one is gradually going upslope.

And that's just what lies at the "feet" of these monsters. Once one comes up to the mesa/plateau proper, elevation gain is swift and steep. Think Mesa Verde, for example.

Ok, now deep within these mesas, which are primarily sedimentary limestone and sandstone rocks, are oftentimes cave systems. As what was once BENEATH the swamps of Dagobah began to rise up (tectonic forces, remember?) cracks were created due to stress. Swampwaters began to seep into the cracks, in turn gradually hollowing them out and creating over millions of years the twining passageways of the system. Ground keeps rising, cracks still there, water is still at "swamp level," but still seeping in through the base or below, to create yet more of the system.

So it's an ancient layered kind of thing, with the "younger" passageways near the bottom (closer to the base level of the swamps surrounding it) and the older ones nearer to the top. Horizontal passageways are connected by vertical cracks, some hairline thin, some wide enough to wriggle through. (Here, think Mammoth Caves.) Surface erosion at their tops can expose a couple ragged layers here and there, with some of the upper caves thereby showing. (Think what the Anasazi cliff dwellers put their villages inside of, in Mesa Verde.)

The mesas and plateaus which hold these ancient systems are GINORMOUS folks; thousands of square miles, with the plateaus being bigger than the mesas. So don't think you can just easily stroll up to the top...