Is it just me or is there a new update to Cities: Skylines that makes all your buildings look medieval? I don't really like the look of them and I don't know how to change it. Is there any way to turn this off?
If you're talking about buildings that look like this:
Then you're seeing the European style buildings introduced in version 1.1.0 of the game, released 19th May 2015. You can see the patch notes here
These buildings are what the developers call a map theme (or biome, they used both in the link above), meaning that whether you see the original or European style depends on the map you are playing on and cannot be changed for that map.
You can see this information somewhere on the map selection screen, before you start your city (I'm not in a position to start the game for a screenshot and didn't quickly find an example). I do not think there is a way to change the theme of a map either before starting or once your city is going.
In addition to DMA57361's answer, if you're willing to go the mod route, there is a mod that makes the building theme a function of a city/district policy instead of the map biome:
The mod maker has even made the mod open to additional "themes" that can be created by users. Currently, there are three user made themes along with the two included in the vanilla game: