Recursion and attrition.
Most of its allies cost 2, letting Unexpected Help and Down the Alley put them into play for effectively no mana spent on the ally itself—while reusing or replaying their Play effects.
The discard pile is a resource: Box of Spirits borrows Play abilities from it, while Miyuki, Crazed Summoner and Unexpected Help give access to discarded allies.
High-impact payoffs like Hordecrusher and Behemoth can be played cheaply or free once the discard fills.
Mantis Shadow and Draining Pillar help filter bad Horde draws.
