Is there a way to reduce the amount of defeat casualties suffered after an attack?

If you are being attacked by someone much stronger, evacuate your troops by sending them out to gather.

But if you want to fight it out:

* Maximize your watch tower. It kills without you losing troops.

* Maximize your fortress.

* Use a defense bonus buff.

* Make your legions as strong as possible by:

** upgrading training grounds

** training the highest tier of troops you can

** upgrading your heroes. Articles are written on the most effective strategies for this.

** equipping troops with items you craft in the blacksmith shop. The site has an article on the blacksmith shop as well.