Edges with mean crease set to 1 are still rounded by a subsurf modifier I am a complete beginner in blender, so my question may turn out to be trivial.

I have a cylinder with some square-ish elements slightly carved into it, as you can see in this screenshot



All of the edges of the squares have a mean crease value of 1:

However when i apply a subsurf modifier the result looks like this

.

What should I do to keep the edges straight when applying the modifier?
Make sure the inner edges are creased at the corners as well. For example:
With only outer edges creased

With inner edges creased

As a bonus, you might want to drop an Edge Split modifier on as well to get your object smooth and remove the stretched shades that creasing and subsurf will produce. Also see How to smooth shade an object while retaining hard edges?.