Please try a couple of things for me:

1. Disable the "Systemintegritäts-Schutz" feature in El Capitan and see if the game runs for you.

2. If step #1 doesn't work then try this:

1. Open System Preferences and go to the “Security & Privacy” panel.
2. Click the lock icon and enter your password to enable changes to the options in the panel.
2. Switch to the Privacy tab and scroll down to “Accessibility”.
4. Tick the checkbox next to each game-app to give it permission to override hotkeys.
5. Quit and relaunch the game app if it was already running.

3. If step #2 doesn't work then try this:

Right-click (or Ctrl-click) the game and select "Open". This should only be required once, and in the future the game should launch normally.

Alternatively, you could see if the game works when you install it via the Mac version of Galaxy but this is entirely optional :)

If none of those steps work correctly, please send us a Support ticket by following this link:

https://www.gog.com/support/contact/technical_issues_with_games/dd_stronghold_kingdom_simulator

and select "launching issues" from the dropdown menu :)