The Fifth Deployable Defense

[napoleon boneaparte]

Ok, this question may be utterly ridiculous for some but I still haven't found the answer after one year of playing and enjoying ETW. During deployment phase, most infantry units can deploy defenses that will provide some aid during the battle. So far, I've seen and used the trenches, cheval-de-frise, the artillery sandbag thing and the all POWERFUL fougasse. But in the deployable defenses bar that appears just slightly above the right side of the battle HUD, there are actually five defenses options rather than the four mentioned above. Does anyone know what the fifth one (actually, it is second from the left of the bar) is actually?

[Primergy]

In the files you can find some hints. Atleast for sieges, things like a sap, ladders and petardes were planned.