Per Deepflame in discord: https://discordapp.com/channels/125011928711036928/360712420240523264/699222772597129238

The idea was for them to become items you would insert into the doorframe So you could upgrade doors to different materials And damaging doors would just break the item, rather than the whole doorframe There's, I think, no programming lets to do to enable doors to work like that Just needs an inventory component with owner limited access, and proper xml definitions set up The facade system was introduced for that purpose too So you could make "metal door" as an item, and it would fit into each doorframe, finding the right shape So you wouldn't have to make "round wooden door", "square wooden door", "differently rounded wooden door"

You need to do the following steps:

1. On the door frame block, add an inventory component with a constraint to only accept door items.
2. On the door frame block, add the visual inventory component with matching facades to assign door frames to the right attachment point etc.
3. Make door items durable items.

Also we never resolved what would happen during construction, and importing pre-0.7 grids and saves All castles would be without doors