What quests become unavailable after "Capture the Castle"?

According to a user here, there is only one side quest that becomes unavailable after completing Capture the Castle:

The only quest you need to finish before Capture the Castle is Warble of a Smitten Knight.

The same thing is said in this thread:

I think its just the Warble of a Smitten Knight quest.

When I did it all that failed was warble of a smitten knight, everything else was fine

Lastly, this thread supports this, as well:

Only "The Warble of a Smitten Knight" as far as I know.

yes just smitten knight quest, is the only one that comes up "failed" when you progress to the next stage

Thus, it looks like as long as you finish "The Warble of a Smitten Knight," you won't miss out on anything by completing Capture the Castle.

It is just the 'Warble of a Smitten Knight' quest.

If you examine the various bodies you come across (if you opt to find Damien) during the 'The Night of Long Fangs' quest that follows on from 'Capture the Castle' you will discover why you need to do 'Warble of a Smitten Knight' first.

Spoiler:

Guillaume dies fighting alongside Damien