Best Defense Achievement/Trophy Guide
The Best Defense Achievement/Trophy can be unlocked as early as Chapter 2. You will meet Bruz in Chapter 2 who will give you tutorials on how the domination system works to make captains allies and one of the tutorials will be the 'Siege Mode'. Basically what you will need to do is reach Level 20 and have a total of 2,250 Silver before this is do-able.
Once you have the above, recruit 2 captains and go to your Siege Tab in the nemesis menu. Now add the 2 captains to the empty spots under Bruz and beside the captain, will be 3 different possible upgrades for each captain. Each upgrade will cost 750 silver and will add a significant bonus to your assault on a fort later on. After you purchase an upgrade for each of the 3 siege leaders you will get the achievement/trophy.