I actually talked to Shik about this a few days ago on whether or not construction sites are allowed to block door ways and stairways. He said it was allowed as they are destructible.
I personally didn't agree with this but I'm not one who makes the rules. Maybe this rule will change in the future :)
******Updated Rule on Construction Sites on NA servers******
NO SIEGE EQUIPMENT OR CONSTRUCTION SITES ALLOWED ON THE NA SIEGE SERVER!!!
-Construction Sites-
Not allowed on any server except the duel server.
http://forum.c-rpg.net/index.php/topic,3702.0.html