"SSR: The 16 Trench counters must form one trench system, with each counter only touching two other counters [EXC: the two Trench counters at either end of the system]. Further, no numbered hexrow on the board may contain more than five Trench counters."
This allows punters to form linear systems (staggered through three hexrows), boxes (not joined at one side), circles (again, not joined), zig-zag systems, etc.
The only thing you can't do is put an enormous blob in the middle of the board to use as your festung.