Tai Lam Chung Reservoir was built in 1951 with a dam that has created a number of small islets nicknamed the "thousand islands". Reaching the viewpoint is quite easy along Section 10 of the MacLehose Trail, about 1.5 hours from the nearest town, So Kwun Wat.
From either Tuen Mun or the Gold Coast, take the 43 green minibus into So Kwun Wat town. You will pass by modern highrises near the coast, which gradually decrease in density into houses as you reach the town.
The minibus stops at the end of the main road facing the Tin Hau temple. Take the path to the left of the temple and continue heading into the forest towards the reservoir. The path is flat although there is a short steep ascent as you near the reservoir.
Upon reaching the reservoir, keep left and head uphill. Do not cross the breakwater to your right. The path gently slopes upwards as you skirt around the reservoir towards the viewpoint.
As you continue uphill, you will notice a bend on the road, an open area with views of the reservoir, and a dirt path emerging to the right just beyond the portable toilet. Head down that path for another 30 minutes or so, and between the 181 and 182 distance markers is a staircase with an obvious label of the view above.
Yuen Long is in the distance behind the hills. Although hazy, Shenzhen's skyline was also visible across the water.
The viewpoint is large although can get quite busy. Some went off trail to a rock further downhill, which is visible from the roped-off observation area.
You can continue down the trail towards Yuen Long, but I chose to return the same way I came to So Kwun Wat. In less than 90 minutes, I was back at the minibus stop just before sunset.