I'm of the view that the engine is considering most obstacles in the archer's line of fire.

I'm not 100% sure if this applies to smaller objects such as trees, but I've done some tests with Sherwood Forresters in ... a forest (remarkably), and outside in a less dense but still wooded area. I think it does apply to smaller objects too, like a big rock, a person etc.

The archers in the trees who had vegetation obstacles fired at a different angle than if they were out in the clearing. They got the most kills if they had a clean straight line of fire or when the trajectory was less than 25-30 degrees and hence they got more kills when there was no object (trees included) in front of them. Again I don't know for sure if they consider the tree to be there oir not. But it's clear that they do consider the embattlement to be there.

The game is doing the type of geometry you are talking about, whether it's graphical only or computational and graphical, someone else will have to answer that.

It's definetly an issue though.

Currently an archer will do more damage (i.e kills) on a flat field, than atop a wall, protected and with a brilliant clear shot directly in front of him, from equal distance. This kind of defeats the whole point of siege defence.

Cheers ! :)