The mages ended the wars by causing the Cataclysm, and now a purple mist delivers mutants every night. Casting the Last Spell takes days of uninterrupted work and somebody has to keep them alive.

Days are for preparation

Rebuild the haven, place walls and turrets, equip the heroes and decide where the weak point will be. Everything spent here is unavailable later.

Nights are a maths problem

Hordes arrive from set directions and your actions are limited. Positioning, area damage and knowing when to give ground decide whether the wall holds.

Roguelite on top

Runs end, unlocks carry over, and the next attempt starts with more to work with. Losing teaches you something specific, which is the mark of a good one.