Is diplomacy necessary?

You can take Diplomacy if you rush to XL and don't immediately return home! You just wander around, replenishing your forces from neutral creatures... it's very profitable to take it for the Stronghold; first, you wander around, they (the monsters that joined through Diplomacy) get killed, and then you can sell the remaining ones. By the way, this can bring me up to 100,000 during a game if I'm playing on XL. It's also useful to give it to the second hero so that he can capture mines without entering the castle to get an army, and if necessary, intercept a weak enemy.

Added 3 minutes later
sell the remaining ones - meaning in the guild of workers.