Glory to Rome
Can a Road or Tower add materials to an opponent's completed Stone or Rubble building when using a Stairway?
Probably yes. A developer says the "yes" ruling applies to the Road and Tower with the Stairway, since the game encourages combos, and two other posters agree. One poster argues the Road's wording ("towards the completion") stops it working on an already completed building, so it is not officially written down.