hi Kiwi64!
There are many ways to get expansions. Below are most of them:
1. As you said winning PVP towers. These towers end every Sunday. So you only get the medals if you are ranking when the tower ends on Sunday. Then just save these up and use them to buy an expansion.
2. Another way to get expansions is to unlock them in the Tech Tree. You'll see them as you progress and once you unlock one you'll be able to buy it for gold and place it on your land.
3. There are also land expansions on the continent map (the tab where you are conquering territories) these you get by conquering all the spots in a territory. Then you are able to buy them for gold in your build menu.
4. Lastly you can get expansions by spending real money on diamonds. I believe the first two are 200, the next two are 400, and all the expansions after that are 600.
Hope this helps!
~Ninga78234