Seems strong but not overpowered, trying it in a test game. Noticed that you spelled vulnerable as vurnable in the unit text by mistake.

i actually already had this exact idea but its kinda hard to implement for some reasons but yeah good idea

What if you added a bunker, similar to like a central command one, but it could only be built directly on cities as a way of protecting civilians, (unlockable after the civilian bunker tech ofc)
because even when turtle shelling in this game, it is hard to avoid deaths even mid game unless nobody pays attention to you and this would be a way of protecting yourself with a tool that provides no value to an actual war effort