It should be fairly obvious at this point that I'm generally a fan of using undead. However, the location that undead are used in is also important for establishing the mood of a session. As I result, I'll be talking about some key things to consider in such a case and focus on the difference between a location that is empty and a location that is dead.
How Is Dead Different from Empty?
The difference between these two words and their meaning is important, particularly when dealing with a location that was hit by a catastrophe (large undead typically result from a catastrophe). If a place is empty, it feels like no one was there before. That sounds obvious, but it is distinctly different from a location being dead. A dead location was once alive and there should be evidence that suggests what the place looked like in life. Depending on the location (even the amount of time passed since the catastrophe occurred), the impression and approach will be different. If the city has been abandoned for centuries, the grandeur it once could be gone and torn away by looters. On the other hand, an area hit by a fresh catastrophe could look exactly the way it looked before but lacking characters. Each of these will lead to a different mood and overall different feeling game.
I don't have anything against the idea of a location being empty. A small area hit by a catastrophe (giant magic accident, maybe?) could have been abandoned by even the animals that would normally be there. Such a situation can be very eerie for the players, especially if they are used to bustling cities. Another example could be a mostly empty and difficult to inhabit place (extreme desert or arctic). The emptiness in these cases would make finding something that much more exciting, even if the find is rather mundane by the standards of a city (e.g. finally seeing an animal or a person).
How to Create a Dead Location?
If you want to make a location that feels like it was once alive but then died, I feel that you have to consider what it was like in life. The easy way in such a case is to create the location as if it was alive and then go through the steps that lead to its death. A city that died from disease may look completely different from a city that died due to an undead army, a regular army or simply fell into disuse. The process that would be applied to the city in each of these cases would most likely be different. It will take more work than just coming up with the final product but doing so helps me come up with more details as well as flesh out the location. It's important to do the same with special rooms. Is the dust even or are there rather fresh footprints and drag lines when entering an old room in a tomb? Are there scorch marks from torches on the wall? What's the smell like? Are there nicks in the wall from furniture that was moved? Scratches in the floor? Broken pieces of pottery on the floor (broken during quick looting)? Are the people who lived there still there (body in the corner or as skeletons)? Are there the broken remains of 4 bunk beds but only 3 skulls on the ground?
Applies to Characters Too
The same kind of approach works for characters too. If you have an evil character, I find I achieve better results if I start with a character that is good or neutral and then apply a process. This way, I'm forced to write the backstory and really think about the character. I also like to do this for characters that are older.