Unlike other games it allowed for a grid-based free placement of buildings, items, decoration and pets.
Personally, I'm not a fan. I like the pieces, the ponds, the archery stalls etc. But overall it was too restrictive in a way that made it feel hard to design something nice. No hideout will look as good as a dev-designed castle.
And of course, a lot of the treasure you find (which AC has steered towards as a thing to hunt down) will be resources.
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I would have preferred a system a bit like the others, where there are places for everything, the dojo goes there, the smith goes there, and you can choose how they are decorated, connected (maybe swap with another building). The reasoning could be you are rebuilding a burned down castle. This way, the decoration could be more hand-placed, giving more detail without having to conform to the grid system. I still get more customisation than in other AC games, but it still looks good.
An example would be there are ruins in the east of the hideout, and you can use resources to build a kakurega, as you have unlocked different wall and roof styles, you can choose them. Then you want to connect it to the Hiroma with a long engawa. The details, such as flowers, rocks, trees, grass will auto generate, making it look really detailed and nice, but they can also be swapped with items you've found.
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What do you think about the system? Did you use it a lot? Did you create cool hideouts?