This sort of falls half way between my old
barricaide style wall and a wall Eero came up with. Presenting: Anti builder wall 2!

From the front it looks fairly unassuming, just a shorter wall, untill you realise...

The gap between the plates allows you to shoot at the feet of incoming Engineers/bastards while they are unable to return fire. The slanting of the plates also means that it is not possible to get a grenade between the plates (95% of the time) without crouching a couple of meters away from the wall. One issue I didn't test with this was how effective rocketeers are against it. From my tests using blocks as a dummy, I found you had to hit pretty high on the lower plank for it to effect the person using the wall as a bunker. (Did I mention that the wall can be used as a bunker? :P)
The angle of the top plate also serves to prevent people from SLAM stepping up the wall.

From the defenders side however, it is easy to just shoot through the gap at the feet of any engineers, on top of that, nearly any grenade thrown at the wall from the atackers side will get caught by the wall and roll outside.
The design of the wall is still not quite complete, I still need to do some fine tuning to get the angles and relative positions of the two blocks right.
Thoughs? Ideas? Complaints?
(Also, Credit again to Eero for the concept)