Anti-Magic Garrison
An Anti-magic Garrison works just like a normal Garrison, with one big exception-- heroes standing on it cannot cast any spells, neither Adventure spells nor Combat Spells. While this makes them more useful than their normal counterparts, they will still most likely not be of great use.
This means that not only can you leave troops here for defense, you can also station a Hero at this garrison by simply having him stand on it without placing his army in the garrison. If he is attacked, no magic can be cast during the battle.