World Conquerer 2 and SAS: Zombie Assault 3 are clearly games made with small teams of developers who know what they are doing. When you say “similar to this”, you are not going to manage anything similar to that without finding a whole team to work on the project.
I think a small tower defence game could be realistic. However, do not open Kingdom Rush or Cursed Treasure and aim to make a game like that.
What you should do is first make just a basic tower, a gold/money amount, the ability to place towers and one monster type that follows a fixed path, that takes damage from the basic tower when in range.
Once you have that set-up, then you can start expanding the game. Maybe by adding a second tower type or monster. After you have implemented some basic things like this, then you can start thinking about proper features and what direction you would like to take the game in.
Whatever direction you go in, don’t hesitate to ask for help on the forum. Making a game is really hard and you will come across tons of problems, but you should definitely follow through and keep doing the project until you have a fully working game. You will benefit a lot as a game programmer from doing so and you will have a cool game to play and that others can play.
thx in advance