Depends on what you want the unit to do on the wall - walk around fighting other enemy units on the wall, fire arrows, get hit by enemy projectiles, etc. There's a lot of functionality that would need to be added, and right now it's low priority. The long wall segment can be upgraded into a gate. This isn't possible yet, but it's on the to-do list. As far as I know there are no plans to include postern gates, although the functionality would be similar to standard gates, so they are possible.