Well, ideally, eventually you'd work all kinds of ancient weapons and tactics in. Caltrops and stakes both are perfectly valid ideas. I can't really say how often something like a caltrop was used, can't think of any instance offhand, stakes and ad-hoc fortifications were far more common I 'spect. Technically, different ones loaded dead bodies/cattle into trebuchet and catapults and lobbed them over city walls over the years, but that doesn't mean it was particularly common.
What all this stuff would boil down to is making WB more tactical, with a more involved pre-battle phase. I personally love all kinds of complexity and realism, so am all for this kind of stuff. The problem is WB isn't, at present, an ultra-tactics oriented game. It's more of a FPS with the trappings of larger scale warfare thrown on. This could be remedied easily enough, just depends on what TW really wants to do with the game and if a majority of people want a "tactical medieval FPS".
I kinda look at this thing like a medieval BattleZone, as is, if anybody remembers that old game. It'd be awesome to make it more complex though.