Note that like all troops that prioritize defenses, Stone Slammer do not consider the Clan Castle to be a defense regardless of whether or not it contains enemy troops, but do consider the defending Grand Warden and the level 12 Town Hall (if its Giga Tesla has been triggered) to be defensive buildings. This is true even if they are under attack by enemy Clan Castle troops, heroes or Skeleton Trap skeletons. Stone Slammer prioritize defensive structures above all other targets, and will bypass all other types of enemy buildings and troops while any defenses remain on the battlefield.The Stone Slammer is the third Siege Machine you unlock, available when the Workshop is upgraded to level 3.
