Surely artillery should be able to pivot though to draw line of sight ?
The narrow arc and limited line of sight will see my poor Trebuchet resigned to the display cabinet

I don't think artillery can occupy defensible terrain, the page and rule quoted above is in the senerio section and is very general. page 52 says that a formation may enter a defensible terrain feature at the end of its move phase, since artillery can't move it could not occupy terrain. This just seems like common sense deal to me, artillery in the forest this is doesn't make good sense. This seems like a definate exploit of the rules to me. A way to give artillery more arc of site like it needs to be anymore deadly.

Sildien

Unfortunately narrower than most. The way it works is that it can only shoot at things in its 'front arc'.
Imagine joining opposite corners up across the diagonals and then extending these lines out away from the base. The front arc is the area in front of the Trebuchet between the two front lines, and there are also two side arcs and the rear arc. There is a clear diagram on page 25 of the rulebook.

There is one way round it though. The base sizes given are minimums. This means you can put it on a base larger than stated if you wish. So you could make the Trebuchet base wider but keep it at the minimum length (or depth whichever way you look at it). This would then give you a wider front arc.