Maybe it is only me but it seems that the Flyingcastle would be best as an Early Unit with the Prerequisite Mysticism or Monarchy...All depends on if the Unit is added to games with "usual" Units as well or if it is added to a Special MOD that uses ALL new units of a specific nature like an all Magic MOD.
...it is still a Castle (Fyling or Not) and generally speaking about Castles, they fit better around the Middle Ages but this is a Fantasy Unit and therefore Not subject to basic or "Normal" settings. As a Land Unit, I set The Victory Sound to reflect the essence of what would be heard from a Castle during the Middle Ages.
Personally, I am using the FlyingCastle as a Land Unit with the following Editor settings:
Prerequisite-Mysticism
Class-Land
UNIT STATISTICS
Cost-10 (100 shields)
Moves-3
Zone of Control-checked
Attack Str.: 7
Defense Str.: 6
Operational Range-3
Bombard Str.: 8
Bombard Range-1
Rate of Fire-5
HP-4
UPGRADE TO: Siege Cannon
Required Resources-None
UNIT ABILITIES
Blitz
All Terrain as Roads
Radar
Ranged Attack Animation
Lethal Land Bombardment
Lethal Sea Bombardment
AVAILABLE TO: I use European Countries but you decide.
AI STRATEGIES -LAND
Offense, Artillery, Explore
STANDARD ORDERS-ALL Checked
SPECIAL ACTIONS
Load
Pillage
Bombard
Upgrade Unit
Yes I have this Unit set Powerful but it is in an Extreme MOD with Very Strong Units.
Note: You can set Rebase with this or any Land Unit and they will Rebase to Any of your Cities from Any LAND area. It is an OverKill Ability if anyone desires it.
All things considered, the FlyingCastle is a very versatile Fantasy Unit...Have Fun with your personal Editor Settings.