This is one of the highest recommended expansions for EU4, and for a good reason- it boasts a lot of improvements to war and it's related systems. It is necessary however for one key feature: transfer occupation. It will change your experience with the game a lot. Other main feature would be the League War, which will make playing in the HRE region of Europe really interesting. The other features are a nice addition as well. (Templates, Sortie from siege, etc.) -1 star for keeping transfer occupation in a DLC however.