I completely agree with you. It's nonsense that we don't have larger maps. Even when building a small castle on purpose on a large map, I still get to be able to target my enemy castle, at least with mangonels. The best solution would be to have 600x600 maps, but that's not an option.
That's why I often, when making a map, start making it with in intention - to be a 2 vs 2, or 3 vs 3, or so on. This way I can make sure that enemy castles are going to stay apart. Also, having oasis behind my castle, to have my workers and stone quarries protected, would be cool, but that too pushes enemy castles closer to each other. Also, having a marsh or a mountain between the enemies can help too, and even make the multiplayer game more exciting.
Unfortunately, I'm afraid that we are stuck with obvious answers such as this one.