Hi

I am designing a puzzle game for the iphone and was wondering as most puzzle games consist of the player progressing through multiple levels. You see for example Angry Birds has over 100 levels.

Once the basis of the game is made, how do developers typically go about building their levels?

Do they generally build them from scratch each one more or less, or work of their own template or have some other method which they use to tailor these levels?

I imagine building so many levels is a long process, certainly if building each one individually. Do they do this, or have a method which speeds it up once they have their basis?

Thanks.