The exclusive fullscreen mode was messing with other apps on my computer, so I poked around for awhile and figured out how to fix this.
I found that the option to change to windowed fullscreen mode is only available in the "Startup Settings" tab which can't be accessed from the game. I managed to figure out that these settings are normally accessed via "start.exe" which we can't use with this "unofficial" copy of the game.
Thankfully I found a workaround: the game knows if it crashed last time it ran, and gives the option to open the startup settings tab on the next launch. To force a crash you can open Resource Monitor, open the CPU tab, then find AGE.exe and right-click and "end process". Next time you run AGE.exe a popup says the game crashed last time it ran, and asks if you want to change the startup settings. Hit "Yes" to open the startup settings.
Since the options window is still in Japanese though I instead devised a way for the layman to enable windowed fullscreen mode by editing the game's option file:
- Run the game at least once using Japanese locale.
- Open Windows Explorer and in the navigation bar paste %localappdata%\Eushully\天結いキャッスルマイスター
- Edit SYS4REG.INI
- Find the lines "VirtualFullScreen=0" and "AspectMode=0" and change them to "VirtualFullScreen=1" and "AspectMode=1"
Now the game will use windowed fullscreen mode using scaling instead of exclusive fullscreen. You can't alt-tab out of it normally unfortunately, but you can use alt+enter to change between fullscreen and windowed on the fly.