The Berkhamsted WW1 trenches are a strange and moving woodland survival: practice trenches dug for soldiers training before the Western Front. They are not monumental, which is exactly why they work; the shallow earthworks make the war feel suddenly local and practical.
Go with a map and realistic expectations, since these are remains in the landscape rather than a museum site. The walk is most rewarding if you already like quiet historical traces and do not need interpretation boards at every turn.