Counter: Fortress vs Knights
|Fortress vs Knights|
Fortress vs Knights is a counter that uses Fortress's unique ability to go to your hand upon trashing to neutralize a painful trashing Attack. Fortress can similarly defend against Saboteur, Swindler and Rogue. Rats can produce a similar effect, but this is outweighed by the fact of having lots of Rats in your deck.
When an opponent plays a Knight, if Fortress is revealed, it is trashed, and on trashing is sent to your hand. So not only did the Attack do nothing, but it also increased your handsize this turn with a valuable engine part. Against Saboteur, you can also gain a good (if there are any on the board), and against Swindler you gain another Fortress (or whatever your opponent sees fit to give you). Thus the presence of Fortress on the board can be enough to deter players from using trashing Attacks, which tend to be quite weak anyway.
This can also synergize with Secret Chamber, which can ensure that your Fortress is in the right place to be trashed.