The picture above is the making process of map in valve Hammer.
The map is basically a l4d survival map where your are in a forest and there is a barn to hold out in. The radio activated the horde and the survival timer begins. The whole production process of the map took about 4 days. FOUR DAYS just to make that. This little experiment really made me appreciate how much time and effort the mod community puts into making custom content for left 4 dead. Anyway if you want to try out my very basic map it can be downloaded at the link below.