Hi
Here is a standard I am working so far. I haven't worked all the kinks out yet, but I have learned a lot from doing several
large layouts. One thing to consider; you will need a lot more crenelations than you think, so it pays to make them as cheap as possible.
I use 1x2 jumper plates with 1x1 plates. You can use a tile on top, but I prefer just a simple plate with a stud instead. It doesn't look that much different, and it is easier to get your hands on a lot of them. It is very simple and the walls join up seamlessly.
16x16 baseplates are a perfect size to build on with one brick high so you can make rivers, ponds, moats, and other lower elevation items. I use a row of brown bricks on the front edge to represent roads. If another row of houses is facing the first row the road becomes 4x wide and looks like a main road. Otherwise it looks like a back road or alley. I am also thinking about placing the wall back instead of in the center so you can have a larger building area for houses, however this may not work out, as I am still developing how to do corners. I connect the baseplates using 2x4 bricks. Technic bricks can added to the wall sections for added stability.
Some examples from my brickshelf folder you can see once they are moderated:
In this picture the wall is moved back and you have more room for larger buildings.
This photo the wall is in the middle, abd may be an easier standard to use, but you don't have as much room for building.
Justin