Absolutely tense atmosphere, delivered by extremely effective writing. JJMcC delivers a relatively-sparse prose, which smoothly causes the reader to connect the dots, and become immersed in the story with blazing speeds. Not too verbose, but not lacking in detail.
You begin with a near-crippling sense of "oh, how am I going to get out of this one??" as the shells render a hellscape around you. However, the satisfaction of going from helpless to Mark-Watney levels of resourceful is incredible.
Above all: You do not need to be an enthusiast of the World War I era to understand what is going on. The game is written largely with accessible language, and fills you in on any details that support immersion and understanding.