In real war, leaders use reconnaisance units to scout ahead of the main force.
This is done to find out if any ambushes are set up.
Naturally your players will do the same, and that is why you make more advanced ambushes with intelligent troops. (Rather: you script them intelligently). First, try playing the map, and then read the readme.txt file included in the zip file, to read about the differences from the standard ambush to the advanced.

I think you will agree with me that this ambush is a bit better planned, and the enemies are harder to kill.

Get map, zipped