The latter stage construction of the half-plane is indeed quite troublesome... but in any case, Hill doesn't need to do it himself.
Both he and Fran have mobile Magic Towers. It's just that there was no need for them to be released before... now, it's quite fitting.
Compared to the constrained Agleya, Fran certainly prefers being in the half-plane: he still has many books left unread and many magic items that haven't gone through a round of refining!
Although Hill's Floating Castle's defenses are considered tight, the presence of powerful Divine Powers nearby made Fran entirely unwilling to fully showcase his true strength... the deities of this world are really much more unscrupulous than those in Teraxil.
Though he knows that Hill has never instructed him to be cautious while working in the castle, it implies that his grandson has absolute confidence that even if Fran causes some big event, Hill can solve it.
