Stopping castle towers from firing on enemy

How can I prevent the towers of my castles from firing at the enemy?

In custom battle, the castles you're given are quite advanced. I'm trying to do some 3-on-3 general fights, and having it take place in a castle is the only way the generals will be close to one another. (for theatrical purposes, of course)

The setup is Oda/Hattori/Date vs Takeda/Uesugi/Hojo, and I've modded them to all be dismounted and have only 1 soldier (the general himself). In regular battles, the deployment zones are too far apart, and the generals would all be fighting away from one another, but in castles, the freaking towers widdle down the hitpoints of the enemy before they crawl up the walls (interestingly, they always aim at Shingen, poor guy).