The French placed hill E1 (abbreviated as E1) as an important stronghold in the line 2 defensive stronghold behind Him Lam cluster. E1 is located close to 42nd Street, from Him Lam through E1 rushes straight into the center of Muong Thanh. E1 was built by the French into a very strong base in defense like Him Lam. There is a system of trenches around, connecting vertically and horizontally, which is very mobile in combat.
Source: Collected internet.