If you place a lid over a mouth, over a wall, over a mouth, this could be tall.
by huangying

The lid of the border stopped it from getting too tall.
by CLTCRhys

The Desert with the tall Border did not have a lid on it.
by CLTCEden

you need to be tall to put a lid on top of the mouth that stands on top of the border
by Kreinus

There are OPENings where people carry stuff *across* the border in their ARMs. To put a LID on it, some want a bi[g ou]tdoor wall ten feet <tall>.
by vms5q