2wolf64: To get there, you need to direct the movement through the Castle gates and, while flying over them, press SPACE! It works, I've checked!

This is a well-known bug, especially annoying because it is directly prohibited on most Heroes resources, therefore, IMHO, many players here also consciously avoided using it.

And doublecast also works. Checked!

P. S. There are many bugs related to flight in general. Flight can be interrupted not only by pressing SPACE or other keys, but also by an event, a battle, or picking up an item (when a hero, after picking something up from the map, stops near the item. For example, at a garrison, without fighting it. Or directly on an artifact). Therefore, on HeroSortale, this is directly prohibited; the wording is approximately: "It is forbidden to artificially interrupt flight, for example, by pressing keys." The latter means, in particular, that the cell on which the hero lands at the end of the flight must be free. And that makes sense.