The locations run themselves. The Barracks trains guardians, the Armory forges weapons, the Factory builds items, the Mine pays CARBON, and the Tower reinforces itself from the Barracks and fires on its own. Every progress bar is one of those timers filling. You never have to click to keep the wall manned.

Your job is choosing where the effort goes. Three of the four actions are free — what they cost is time, bodies, or a thing you only have one of:

• Upgrade (costs CARBON) — permanently improve a rate or a cap for the rest of this run. This is the only thing CARBON buys, and it compounds, so an early level is worth several late ones.
• Raise (free, costs time) — empties the Crypt: every guardian in it comes back at once. The Crypt holds your dead indefinitely, so none are ever lost, and a higher Crypt shortens the wait between rites. Because one rite returns everyone, raising the instant it is ready spends the whole cycle on whoever happens to be dead — holding on returns everyone who falls in the meantime for the same wait. The judgement is whether your wall lives long enough to be worth banking.
• Strike (free, costs guardians and gear) — kill attackers in the open before they reach the wall. Trades bodies for wall damage you never take.
• Factory items (free, but each is spent for good) — the shelf under the board. Which one you pick is the decision; see below.

The strategy: spend CARBON early, while a level still has a whole run to pay you back, and lean on the free actions once waves outpace production. Holding CARBON does nothing — unspent CARBON is a wave you didn't survive.

The seven items, and when they are worth spending. The Factory builds them on its own and holds a limited number, so a shelf full of items you are saving is a Factory that has stopped producing.

• Wall Shield — the next attacker to reach your wall is thrown back and does no damage at all. Stacks, and never expires: three of them is three breaches cancelled. The strongest thing you can hold when the wall is about to be hit, and worth most against the big attackers, who hit for 12 where the rest hit for 5.
• Rush Lines — every production line that has room finishes at once: a recruit, a weapon, a piece of armor, another item, and both cooldowns refilled. A line that is already capped out delivers nothing, so the cards that actually produced are the ones that flash. The Mine is the exception that always pays: its bar measures the next Mine level rather than the next coin, so a rush hands you the CARBON that level costs — yours to spend there or anywhere else. The Portal and the Crypt cannot be blocked either, so this always buys an immediate Strike and a ready rite. Best used the moment you want guardians out and the cooldown says no.
• Volley +100% / +75% / +50% / +25% — each patches the wall and makes the Tower hit that much harder. Both the patch and the duration scale with the magnitude, on the same rule: the number on the tin over ten. So +100% is 10 wall and ten seconds; +25% is 2.5 wall and 2.5 seconds. Two things worth knowing: the patch is capped at a full wall, so spending one at 100 wastes it — and the Tower fires one shot at one attacker every 0.6s, so once your guardians already kill an attacker per shot, hitting harder kills no faster. Late in a run these are mostly a wall repair — and because the patch scales too, a +100% is worth four +25%s once the extra damage has stopped mattering.
• Free Level — one random location gains a level for nothing. It is the only item that compounds, and the only one with no effect on the fight in front of you. Spend these early, while a level still has a whole run to pay you back — and spend them to clear shelf space rather than sitting on them.