Agreed Lange, using construction sites to block access is also not allowed. That's an old 'rule' but it's good to repeat that here.
Do NOT use siege towers or construction sites to block access to flags.
I hope we can all learn to use construction sites in a fun, constructive way without too much admin intervention. But again, if you see misuse of siege equipment, be sure to 1) explain that it's not allowed and 2) get an admin if needed.