Fukuchiyama Castle?

[mishari26]

This castle is weird, it has the red diamond icon on the map and not the castle icon showing how many Daisho's need to be killed. also, I covered the whole place and couldn't find any Daishos.

Yet there's a legendary chest at the top of its tower that is locked and says I need to defeat all the Daishos??

it's in Tamba region.

[katzenkrimis]

I haven't done Tamba yet, but I've never had a problem finding the Daishos.

They must be there.

Search everywhere, inside and out of every building. And along every walled part of the castle. Look closely on the map to see its exact size. Sometimes they have lower levels and other connected areas.

Hold down V to use the eagle cheat view.

[Cairn.L]

I just completed that castle yesterday, it's probably bugged on your game, try reloading or something.
https://steamcommunity.com/sharedfiles/filedetails/?id=3458315641

[Lukinsons]

you can probably finish it after Yasuke´s private mission almost in the end

and in another castle in Tamba region too

[Shin Happens]

ANY castle that has either an assassination / duel target or some other mission tied to it will not have the legendary chest and samurai daishos available. You need to do more quests until you get the one that leads you to this castle, before that wealth will be "low" and the legendary chest unavailable. Once the quest in the castle is done you can return and THEN the samurai who unlock the chest are present and the buildings with the resources can be unlocked. Before that you will not find the key that unlocks the doors.

It is a somewhat confusing gameplay decision, castles should always have their loot available, even if an assassination / story target is in the castle.

There are quite a few of those locations that only unlock the legendary chest once you have finished a quest, there is also a fort in Iga for example, that has an ally quest you need to do first.