This app was mentioned in 8 comments, with an average of 0.75 upvotes
The little war game series are great fun, not too complicated. 4 of them I think :
Great Little War Game :
https://play.google.com/store/apps/details?id=com.rubicon.dev.glwg
One of my favourites - The Last Warlock
https://play.google.com/store/apps/details?id=com.sonicsloth.thelastwarlock
Your wish is fulfilled
Great Little War game
https://play.google.com/store/apps/details?id=com.rubicon.dev.glwg
European War 4
https://play.google.com/store/apps/details?id=com.easytech.ew4
Fortress under siege
https://play.google.com/store/apps/details?id=com.easytech.fortress
Note:These are just single games from a series. I have mentioned them only because they are the most successful titles of that series.
Maybe the War Game series? Check them out:
I'm not sure there were monsters though... Have played only the first one, a long time ago.
Have you tried Great Little War Game?
Careful, there's a game named similarly already: https://play.google.com/store/apps/details?id=com.rubicon.dev.glwg&hl=en