Object appears smoothed after rendering but not in viewport I'm pretty new to blender and used it to model a castle and print it.
Now I want to extend the model by textures and render it, but even when I render
it with basic material i get some strange output.

Does somebody know my error? I already corrected the normals and they look fine.
you have a subdivision surface modifier (or multiresolution) set to render only, not visible in viewport but active when rendering.



Click the eye to see it, click the X to remove it.

As suggested by Chebhou, the subsurf modifier can also be different in viewport/render because of the subdivision levels number set for each.