it is "lying in wait" not "lie in wait".
It means that the enemy (soldiers) are hiding along the route ready to attack or surprise them.
a surprise attack by people lying in wait in a concealed position.
to use "lie in wait" you have to change the tense.
"Don't send your soldiers on that route. I heard your enemies will lie in wait to ambush them along the way."
"lying in wait" = they are already hiding ready to ambush or known to be already hiding.
"will lie in wait" = it is known/believed/suspected/ pre-supposed/guessed that the enemy will do this or are very likely to do this.