Making a game is more about the mechanics of the game than the art and looks. What you want to be able to test are these mechanics. This includes things like rankings, levels and things that show the player they are making progress (so that if they do want to leave they have to consider the amount of time they spent building up their avatar, castle, etcc....).
I disagree with the rest of our colleagues here saying you can test this idea with a paper version. In order to know for sure you will need someone to create a basic version of the game where you can test a few things:
* retention levels (typically you want to keep an eye on how many players return after 7, 14, 21, 28 days after installation)
* how much you can expect to make on ads.. ( if it's free-to-play focus on eCPM)
* how much it will cost you to acquire users (typically on a cost per install basis)
With these you should know within a month if it's something that can scale.
To get to this point you need someone to build an early version of the game for you.. and well that costs money. Trying to learn to build a prototype, when you have 0 game development experience, is a waste of your time. If you do want to focus on something I'd recommend researching what the benchmarks for RTS games are on the bullet points above.
The cheapest way for you to build your game is to hire a small game dev studio. Make a deal with them where you will cover their costs for building an early version of the game and will then give them a revenue share portion of what you make over time. Make sure they dont spent too much time on art. Again, test the mechanics.
Your game should be ready within 3 months and you should have ~$1-2k to test with paid acquisition so that you have some good sample sizes to see if its successful. Good luck!