Upgrade cards can be placed on the respective tower and grant a permanent bonus to them (possibly stackable)
A few ideas:
Dark forged towers:
Dark Black Widow - grants her shadow blades with a chance to instantly kill small enemies (mass/normal)
Dark Hitman - grants mark of the assassin increasing the target's damage taken when hitman performs a crit.
Dark Frog - poison also destroys X armor over time.
