When you unlock your first purple hero, you are adding one more hero to your tavern for other uses. When sieging certain cities (resource city), a player will need their subordinate heroes to hold certain cities. The option “assemble” allows the player to choose what heroes they want to use to hold a city. You want to use this option very strategically. Assemble your heroes to cities that are choke-points so that they will slow down enemy troops. You also want to stagger your heroes meaning that you put heroes behind the choke-point cities, which slows down the “assault” option even further.