There's a great, big bat cave... with bats! The off-trail trek to the giant cave is completely optional and should probably be saved for last on this eventful day hike. Unlike the other knife-ridge hikes around Taipei, Huang Di Dian doesn't follow a convenient loop and the best route to take is from west to east, starting with the wilder, less popular West Peak trail and then hiking through to the East Peak ridge. The constant up and down scrambles put this trail on a similar level as Wuliaojian, but it still isn't quite as challenging as Taipei's most famous day hike.
Be warned, there are two different sets of steps leading up to the West Peak trail. The first one you come to on the road from Shiding is not the one you want to take as it's two hours of unrelenting hell. (Whoops.) Bypass this illusion of beginning and continue up the road until you see a second set disappearing into the woods. This second set will get you to the trail faster and has a nice little pagoda where visitors can take a short break to enjoy the view. (I think I remember an abandoned building nearby as well as lion guardians marking the entrance, but my memory may be getting fuzzy.)
Once on the West Peak trail, things get more fun with a nice dirt trail that climbs over roots, around trees and up steep, smooth rocks. It's a nice quiet trek as most tourists don't wander onto this side, preferring instead to climb up the East Peak Ridge, get their pictures and then return to the safety of civilization. After an hour or two, the trail crosses another set of stairs heading down, this is the East Peak entrance as well as your exit. You'll start seeing more people now on the way to East Peak's famous bald ridge. The best views are captured from that side.
On your way down the East Peak exit, there is a VERY small trail leading off into the woods that is marked by a set of steps and a pile of trash (unless some good Samaritan has come along and cleaned). Beyond the empty water bottles and candy wrappers, this trail leads to Huang Di Dian's bat cave, which rests just below the East Peak Ridge. "Trail" is used figuratively here as it's more like a game of "hide and seek" with decaying tree flags.
Our trek to the cave was very slow as we were constantly searching the surrounding area for signs of past traffic (flags, ropes, etc.). We were basically picking our way through an overgrown path that had never been very large to begin with. Just before the cave, it turns into a narrow footbridge against a cliff supported by the thinnest line of vegetation growing against the rocks. Residents of the bat cave don't need to worry about hoards of tourists, as this precious little path could easily kill them all.
Our trailblazing skills were ultimately rewarded though. Soon, the giant cave came into view and we carefully half ran/half scrambled up to the mouth, zig-zagging between a few well-placed trees to keep from falling back down the cliff. Once inside the mouth, we were pleasantly surprised to find bats sleeping above us in the cave's ceiling. (Everything I'd read up to this point had said that the cave was deserted.) After some pictures and a dare-devil descent into the cave from my climbing partner, we slid back down the mouth and headed back the way we came. Familiar with the way and racing a setting sun, we made it back to the main trail in about half the time.