The door is the health bar
Monsters do not need to catch your avatar. They break the house door. If the door falls, the run fails. Upgrades raise its maximum health, and play reports describe early doors around 800 HP growing into the thousands, with Golden Door and Diamond Door stages on long runs. Doors have been pushed past level 12 in extended matches.
Door level is also a lock. A level 3 door is the reported gate for several higher tower and utility upgrades. If an upgrade button will not light up, raise the door before you move the tower again.
Repair
Stand at the door and use the manual repair prompt during combat. It is the emergency button when a boss sits on the gate. It does not fire instantly every time, so pair it with prevention:
- Kill enemies before they reach the door.
- Place a Medical Base close enough that its healing beam can touch the door. The beam restores door HP over time only while the door stays in range.
- Upgrade the door before a named boss wave instead of spending the same cash on one more low-level Crossbow.
One door or two
Easy mode houses have a single entrance. Hard mode houses have two, including a larger desert layout. Both doors need levels and towers. Leaving the second door at its starting tier is how Hard runs end.
On a two-door house, split income instead of maxing one side. A Medical Base only helps the door inside its radius, so one base does not cover both entrances unless you confirm the range in that house.
Boss pressure
Iron Fist walks to the door around Easy waves 7 to 8 and is manageable if the door is already level 4 or higher with upgraded Crossbows. The Golden King on Easy wave 15 is reported at about 30 damage per swing. Hard bosses hit much harder: Thorn Warden is reported around 25,000 HP with self-healing, and the Cursed Priest is reported around 45,000 HP with fast door damage. Those Hard numbers are play reports, not a developer sheet.
Set Mortars and advanced Turrets to Strongest so they shoot the boss instead of the crowd at the gate. Keep repairing while they do it.