it should be possible to select multiple buildings and give them orders, like double-clicking on a barrack to select 3 barracks, then click once on archer and build archers in all 3 barracks. that's how it works with a summoner, why not for barracks?
some orders should affect all buildings, like producing units or setting the meeting point, some not like upgrade training field.
selecting should be possible with a double-click or shift-clicking to select additional building of the same type.
marking a group of buildings with ctrl-1 should be possible