城門水塘鐘形溢流口，當水塘溢滿的時候，多餘的儲水就會由這個水碗排到下城門水塘。這是一項水務設施，通往那碗的小路是不能進入的。但在閘外拍到小徑穿過一個圓形水瀑布的景象。沿城門水塘車路漫步，抵主壩後下走百步梯，一睹萬馬奔騰的排洪出水口（有水的話），再經鐵棧道探遊下城門郊遊徑，以遼闊草地為草蓆，到達下城門直出大圍美林鸷散隊，途中遇適當地大休，行程約三至四小時。In 1933 the Shing Mun Reservoir was built to meet the increasing demand for fresh water due to the urbanisation of Kowloon. The reservoir was completed in 1937. It was once named Jubilee Reservoir (銀禧水塘) to celebrate Silver Jubilee (1935) of King George V of the United Kingdom.The local inhabitants were resettled in other parts of the New Territories, and now some of the old villages are submerged. The remains of other villages and houses can be seen in the woods on the side of the reservoir. The remains of Gin Drinkers Line on the nearby hills show the defences of British force against Japanese invasion during World War II.It is possible to see many troops of macaque monkeys around the picnic sites and in the woodland areas. To preserve the natural environment of the reservoir, its surrounding area is managed under Shing Mun Country Park. Two walking trails, Wilson Trail and MacLehose Trail, cross at the side of the reservoir.