How many stars of destiny are needed before the castle reaches level four? I seem to remember being able to recruit Leon and Pesmerga right before going to see the Dragon Knights. I could be misremembering but reaching the Level 4 castle before then will make recruiting Leon easier in my current playthrough.

I believe the castle upgrades with 90 recruits.

However because Viktor leaves I do not think you can have enough straight after Neclord's Castle; but after picking up Fuma and Kreutz at the Dragon Knights Fortress you should have enough for Level 4 before doing the Seek Valley mission.

*goes back and checks recruitment guide* Looks like if you get all of the recruits as soon as you can, you have exactly 90 after Hix and Tengaar join, after Neclord. Good, this makes Leon much easier to get.