This game got me into modding and level design. Oh, the hours I spent making my own maps! The level design is the best of any FPS ever. Remember that it came out in '96 and had realistic city scenes when competing with the likes of Doom and, later that year, Quake, both of which I have done level editing for also. Now every 3d Game that depicts cities, owes a great deal to this game. If you want to get into level editing, this is a good place to start, especially due to the fact that the source code has been released.