Feats of Strength increase difficulty. In Campaign mode, they can be activated in the lobby options, and each one provides a 20% experience bonus. In Survival mode, a single Feat is periodically activated as the player progresses until all of them are activated. The Feats are:

Beast Tamer Beast Tamer – Enemies attack twice as fast.
Hardcore Hardcore – The player cannot respawn during the action phase unless they equipped the Phoenix perk. When a tower is sold, only half of the invested resources are refunded, as opposed to all.
Hydra Hunter Hydra Hunter – Enemies regenerate 150 hit points per second.
Impeccable Accuracy Impeccable Accuracy – Enemies move faster.
Titan Slayer Titan Slayer – Enemies have 50% more hit points.

Sanctum 2