To be honest, I prefer the completely free building of castles. Being restrained to a "proper" castle (e.g.: All walls aligned. Automatic towers at the outer walls) limits the possibilities to build creative stuff. You sometimes build castles underwater, castles hanging around a cliff, etc.

I think a special "castle-building-hammer" could be introduced, or castles become a submenu of the normal hammer. Whenever you activate that hammer, castle building options are made available depending on how you stand in the castle. The options you described (outpost, castle extension, etc.) are available, but custom parts - like just a left wall in the middle of nowhere - can also be placed. More importantly, castle parts also need to be placed behind existing buildings. It's really annoying that you cannot extend your castle in CR sometimes, because you happened to build an anvil somewhere.

I'm also a big fan of slow castle construction that needs lots of materials. We sometimes spent nights just building castles in CE/CR. But I guess that can be optional and would be used in hardcore settlement scenarios only. Construction materials are still possible in the engine, but currently the rule is not used. We might revive it if demand arises.