Rook Enforcers completes the archetype line with ranged specialists. It needs Rook Brutes.
Rook Enforcers is entry 12 of 17 in the Ringleader branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £8,000, 100 Metal, 50 Silk. Before Rook Enforcers unlocks you need Rook Brutes.