How about a two-for-one today? Perhaps something from the RTS tower defence and city builder departments? Oh, and we'll throw in a medieval flavouring too, on the house. With all of that mixed together and wrapped up nicely, you get Bergfriend, a game in which you build medieval castles to your own specifications, and then defend them from rival knights who want to take them from you.
In Bergfried, you don't just plonk down prefab bits of a castle and call it a day. It's a lot more granular than that. You're the own that draws up the literal castle walls. You decide where your tower goes, because every castle needs one of those. Beyond that you also design the town your castle rules over, where all your subjects go about their day (and gather up resources so you can build more castles). So yes it's a city builder, but a very customisable one. One of the key elements in designing your castle is that you'll do so "on a unique grid that captures the irregular shapes of European castles from the high and late middle ages." History!
Then comes the other half of the game. Once you've built your town, you have a bunch of knights who want to take over your castle town for their own (unspecified) nefarious purposes and deeds. This looks fairly involved too, allowing you to position your troops as you like, deciding who and when they attack, all in the hopes of defending your castle. Judging from the trailers you can even command specific knights to do particular things, rather than just instructing entire units.
As it currently stands, Bergfried doesn't have a release date to speak of. However, you can sign up to playtest the game if it takes your fancy, and if not, you can always just wishlist it on Steam.