Thanks, guys. The construction was not represented on the map, but it completed eventually after an event where I had to pay 100 more, for a total of 500. As a converted tribal I had lean counties, so it would have cost me 600, but I imagine 400-100=300 for a trustworthy architect is a really nice thing when you already have several holdings.
Now I wonder:
1. What happens if you don't have a demesne county with a spare barony slot?
2. Do tribals get this event and actually receive a castle?
3. What if I actually ordered a normal construction in the meantime and exhausted all my slots? Would I get an extra castle slot the same way the military order events may very well lead to a lot of such extra castle slots in k_Jerusalem?
I can confirm that the event doesn't check for standard restrictions regarding building type. Thus, I now have a county in which there are 3 castles and only that. One's mine from a converted tribal settlement, another is owned by a military order and finally the one from the architect.