The Amp Spire is a tower in Sanctum 2 that increases the damage dealt by nearby towers, unlocked at Rank 12. The damage increase does not stack with other Amp Spires. The Amp Spire does not damage by itself.
Because the Amp Spire can affect multiple towers, spare resources spent on overcharging the Amp Spire may lead to a larger overall damage increase than overcharging the other towers, depending on the number and type of towers affected by the Amp Spire.
At levels 1 and 2, the Amp Spire's radius covers up to 8 of the closest towers; at level 3, it affects up to 20.