Thanks to all of you, I've come up with an idea... an arena. Placed comfortably outside a base but still inside the red circle.. so with a 40 meter diameter base (first workbench), and giving the trolls a comfortable swinging space; you're left with about a 40 meter diameter max arena size.. make it a smaller octagon say 32 meters in diameter - gives room for a small archery tower in the middle for use against some mobs; plenty of kiting room vs anything else. Using the 4 meter log beams to make the perimeter, then the X piece 'tank traps' and plenty of room to kite and dance.

Will test, will report results later :) Thanks again everyone!

[edit]: Ok, forgot, flat areas that size are rare.. except for one place: stone circles in meadows... hmm...

[edit 2]: Ehhh... it sorta works and sorta doesn't... problem is, for one thing, the outer perimeter needs to be *very* level, otherwise some shorties will get past the logs. Also, the trolls seem to swing at the stone circle stones for some reason, occasionally hitting one of the X pieces. I tested with a 'shrunk down' 20 meter diameter 'circle' which means you really need a *very* level 24 meter diameter area - may be hard to come by at a typical base site.