The solution is pretty simple:
*add an invisible collision mesh to the open top of the trench
*it's one-sided, so fighters can exit the trench at any time but can only enter at a specific area which can be indicated on the mini map
That way players are forced to make an attack run properly.
Also all turrets should be removed from the trench.
Check out my blog about how wonderful the internet is, called
The Wonderful Internet.
http://thewonderfulinternet.com/