Nosliw and I have made a battlefield/fort world in the aether. I thought since we've made one already, we'd put down some guidelines for the fort world!
I'd say maybe 25x25 to 29x29 for the fort land SIZES. Then like 30x50 for the battlefield. Allow me to get more in depth.
On your fort land and ONLY fort land, you can make whatever you want, even just a piece of dirt.
Then, in the big battlefield, you can make siege towers, cannons, WHATEVER they can think of to try and siege the opposing fort.
How you win, is you have to get the other fort's gold block, their flag, if you will. Once you get it, you must secure it by putting it onto your gold block.