Well, acctually I think the tower would break even if it was of metal. Maybe even if it was of diamond and had the same scale compared to Rook's hammer. I mean... he should be able to just push it over with it (if he can't destroy it I mean)... BUT that's not the point...
When the rook walks he should only be able to walk either north, south, east or west. Since that is how the real rook in chess works!
Once he starts moving in a direction he wont stop until he hits void. Any units coughts in his path instadies, including citadel.
Hmm interesting idea. If we change everyone in this matter Queen won't have to be buffed again! She will own everybody.