Levels aren’t made by placing walls; levels are made by planning. Once a level developer is accustomed to the tool-set at hand, and the underlying game engine, emphasis shifts from placing walls to placing rooms, and from placing enemies to designing encounters."
Level design encompasses a lot of different elements, such as lighting and layout etc, but as most games involve (you) the player interacting with levels in one way or another, it really just means planning how that experience works. At its simplest, it involves specific decisions by the level designer which impact the playable environment for the player. Usually it this process begins very early on in the games development pipeline, even before final level/ environment concepts are drawn up. The designer will first sketch some some idea's for the level to use for block out;Usually a top down view.
The goal behind this is that it helps inform the developers how the player can traverse the game space. As anyone who has played a game with flat non interesting level designs will know, this is kinda important to get right!
Through work on our off the map project, I have become familiar with how important the white box stage is when designing a level. As a team we all contributed to this stage and made sure it was spot on before we began finalizing models moving on with the design pipeline.
I found a good video tutorial that discusses level design to a strict timeline day by day, check it out -
Where as, with Darksouls, its level designs are more enclosed, and they more or less guide you specifically to a set destination. Perhaps that wasn't the best example but its highlights what I mean.
Even if you take two slightly newer games, such as Bio-shock infinite, and Dishonored; games I felt had a very similar look and feel, other that the obvious contrast in color scheme. They may have a very similar look and feel to their UI and art style, but the philosophy used in their respective level design with differ hugely.
This want of individuality in design across titles is often very intrinsic to their success. Nobody likes a copycat!