IMPORTANT: These have been sealed off by a tall fence and barbed wire. There may be a way to reach them still by climbing through the forest.
Above Toad Mountain village is a remnant of Taiwan's defenses against a possible Chinese invasion. There are a series of bunkers and a large cave system. To get there follow the path through and above the village and along the wall of the adjacent military base. Past this first hut you will find the first two bunkers cut into a steep cliff.
The first two bunkers have barrel roofs and are largely empty. Perhaps it's those two empty circular sockets above a gaping hole, but I found the entrances very eerie.
Moving along the cliff to the right you'll find another set of bunkers and a small outbuilding. One of these looks like it was lived in at somepoint in the 90's. There's an old tv, some spotlights, a washing machine, and various electrical equipment.
Continuing further you will come to a large cave entrance. Within this cave, which maybe was the start of a planned bunker, there is a tunnel sloping down to another large cave. It's very dark in here and in the bunkers so bring a torch.
It's possible to climb up the mountain here and into the military radar installment at the top. I wouldn't recommend it though as there are probably harsh penalties for doing so. We climbed a little, and ended up in there, as we couldn't get back down. A soldier saw us coming down some steps, but he didn't seem too bothered. The end of the video below shows this particular adventure.
If you're in the Gongguan area this is a fun exploration in a long forgotten part of Taipei. It felt like we were the first to enter these bunkers for many years. Search for Toad Mountain on the Hidden Taiwan Map