Upgrades are a limited budget
You earn a small number of upgrade points across a run. Each point makes one card better. Because the budget is thin, where you spend decides more than how much you spend.
The instinct to upgrade everything is the trap. A point on a card you rarely play is a point wasted. Aim the budget at the cards that carry the plan.
Upgrade your repeat performers
The cards you draw and play in most battles deserve the first points. Your main attacker, your front defender, your key draw spell: these show up constantly, so their gains compound.
A point on a repeat performer improves every battle it appears in. That is the highest return you can get from the budget.
Leave the filler alone
Cards you intend to remove should not be upgraded. The point is lost the moment the card leaves the deck. Upgrade only keepers, and remove the rest first if you can.
Some cards barely improve when upgraded. If the upgrade does not change how the card functions, skip it and spend elsewhere.
Upgrade for the boss
Late in a run, aim upgrades at the fight that ends it. If your attacker needs more reach, upgrade it. If your defense cracks, upgrade the wall. The boss is the test, so build toward passing it.
The boss strategy page helps you read what the boss demands, so your upgrades answer the right threat.
A quick order
Spend first on your main attacker, then on your key defender, then on draw or scaling. Stop when the next upgrade would land on a card you do not rely on. That order keeps the budget honest.
Next: Boss strategy.