It appears that Castle is no longer available in the iOS App Store. If you had it downloaded in iTunes on a computer before the App Store was removed from iTunes you can follow the instructions below to recover the app:
"Sync" Apps via iTunes:
What happens when you need to download a past App purchase that is no longer listed in your "Not on My iPhone" list because it is no longer in the Store, or if you need to restore an older version of an App?
The following instructions will tell you how to "Sync" Apps that are no longer available in the Store, or if you need to restore an older version:
1. On your computer, go to iTunes>Preferences>Advanced: Note Location of ITUNES MEDIA LIBRARY
2. Hook up device to computer with current iTunes
3. Click on Library view so that the sidebar can show your device:
4. Then open Finder or Explorer and follow the path to get to iTunes Media Library. Click on Mobile Applications, and find the app you want. Manage the size of your two windows (iTunes & Finder) so that you can drag the selected app from the Finder/Explorer folder to the sidebar. Hover just above the device info until you see a square around the device info. Drop the app there and it will sync to the device immediately.
Be aware, however, that if you are running iOS 11 or higher you probably won't be able to use the app. If it was removed from the App Store it is possible that is because it was a 32bit app. All apps on iOS 11 and higher must be 64bit apps.
Best of luck,
GB