Trying to make a fort/citadel but I don't think I have the right assets.

Hey everyone, as the title says I am making a battle map for my campaign that is in a fort/citadel. Unfortunately I have the base version of the dungeondraft and I don't think I have the right assets to make like archer towers or ballista towers. Any ideas where I can download a free assets that might have it or is there a way I can use what I have and make it from there?