Bulwark

From HammerWiki

Bulwark

Bulwark Buff Core Ability Learned on RR 45 No cost 5 ft range 15.0s cast 5m cooldown Only usable on Doors Repairs a friendly keep door for 5% of it's total health over 15 seconds.
Details
Usable by	Ironbreaker  Knight of the Blazing Sun  Swordmaster  Black Orc  Chosen  Black Guard

Bulwark

Granted at RR45, it's not listed at the Career Trainer. that said, it may be similar to the skill Deflect Oil in that it's not purchased, the game just gives you it.