I thought a lot of people knew this trail. However it's come as a surprise to everyone I've mentioned it to. I think a fair few people know the main trail, but few people know about the beautiful waterfall and river valley beneath it. And not so many people know that this is probably the easiest place to see wild monkeys in the Taipei area.
The Tianmu Old trail starts from the top of Lane 232 Zhongshan North Road Sec 7. It's easier however to take one of the buses up Yangmingshan, and get off at Chinese Culture University. Here you will find yourself in the US forces housing area. These mostly abandoned white bungalows were built for US service personnel in the 1950's. They have been designated a historical site and a few have been restored (though some quite poorly).
Walk down Aifu 3rd Street, Lane 12, and you will find the well marked trail head. Follow this down to a flat gravel path. You'll eventually come to a junction with a wooden signboard. Take the first right, and at another junction right again. You will find yourself on a rough path descending into the valley. When you emerge from the forest you will be at Emerald Peak Waterfall. This is quite a nice fall with a large pool. However it has a water pipe draped over it, and on one day I visited some garbage was in the pool. A different day I swam in here with no ill effects, but I don't know where the water flows from. The forest in the valley is thick and tropical. It feels a bit like a forest in southern Taiwan. There are large ferns and banyans around. Perhaps this is why the monkeys like it.
After the waterfall either go back up the same path or follow the valley along and take the next path up. Both will lead back to where you started. It is on this small uphill path that I have seen the monkeys. The first time I visited I followed the river valley, where it turned into a narrow hiking trail, and came out in a farm on Lane 219, Zhongshan North Road Sec 7. This route is marked as Emerald Peak Hiking Trail on the Hidden Taiwan Map. There is an easier way which leads to the same road where it makes a hairpin turn. This is marked as Emerald Peak Hiking Trail Easy Spur on the Hidden Taiwan Map.
On the path back up to the Tianmu Old Trail you should find the monkeys. I went on a weekday and found the monkeys around 4:30pm. They were coming down the mountain and eating from the trees. It's rare in England to see wildlife up close, as it's all very shy, so it was great to be able to get so close to monkeys. I can watch monkeys for hours, and find their human-like mannerisms fascinating. They don't find our monkey-like mannerisms fascinating, and weren't bothered at all by me or my friends.
After watching the monkeys you can take the main trail down to Tianmu, which follows an old water pipe (marked as Tianmu Old Trail on the Hidden Taiwan Map). However this trail is long and a bit boring, with tons of steps. Unless you love steps, it may be easier to go back up to Chinese Culture University and take the bus back down.
There are some other trails on nearby Mt Shamao, and the technically off limits Shamao waterfall in the same valley. I've mapped out those on the Hidden Taiwan Map here.