This area's hint region should be CASTLE_GROUNDS as it represents the decider of the castle grounds real location based on age.

We currently don't have a hint table entry for CASTLE_GROUNDS, so you would need to add one around here:

That was an entrance I was unsure of. My logic for choosing market is that the check is on the market side, so you would be leaving market into one of those 2 locations. Therefore, if that entrance was rolled to become an interior, you would reach that interior from the market.

I may have misunderstood the "real" location of this entrance, I see 3 things this entrance could mean:

• It is the north exit from market, meaning that intuitively it is part of market
• It is the south exit from castle, so would intuitively be in Hyrule castle and OGC at the same time, Which is probably where "Castle Grounds" comes in, but this seems unlikely as this is the side with the adult/child logic check and even if it was the idea of having a whole new hint area for a single entrance bothers me, I would seriously consider merging castle and OGC completely at that point but would need to think through the consequences.
• It is a virtual entrance that doesn't actually correspond to the real one, in which case I oopsed and didn't need to give it an entrance key at all.

The trick is to not think of these as "entrances", but instead "logical areas".
In this case, Castle Grounds is the logical area between Market and the child/adult castle areas. Think of it like the other ones like "Behind king zora". It is basically the last logical area before you get to the the market side.

Basically the only time this would come up is with mixed interiors and most likely decouple as well. So like say you're only way "into" HC/OGC, is through the great fairy exit, then it's possible you might have a hint for an interior check behind the exit to market. Having it be "Castle Grounds" makes sense as either adult or child could get to the exit.