As far as I know, every weapon and piece of armor will have kind of a hitbox all of the time. That is to say, you won't be able to swing through the shaft of a spear with your sword, even if it's just sitting idly in ints user's hands, not performing any kind of block. Unless they implement the ability to hack off spear tips.
According to the last devblog there will be multiple stances, each offering a different array of attacks and
?blocking
? techniques. Don't know much about combat in Kingdom Come, but it seems to be sequence based. To my knowledge, combat in Melee is going to be much more fluid. Like Warband, but without the quirks and a bit of icing on top.
