Decreasing Var's on hittest

Im making a defense game, i have the defense (castle thing) on the right, i have my "enemy" going right, part of the enemys code is the folowing:

onClipEvent (load) {
_root.speed = 3;
}
onClipEvent(enterFrame){
if(this.hitTest(_root.defense))
{ _root.speed -= 2
} }
onClipEvent (enterFrame) {
this._x-=-speed

}

So i got my dude going right at 3 pixals every frame, "defense" is the instance of a movie clip on the main stage, its ment to be a trench to slow down enemys, but the enemys speed doesnt change once it touch's "defense"

Is the actionscript have a mistake init? wrong way round? It doesnt report any mistake when i test the game...

Help please