Mainly, the difference being the method of engagement. You guys would appear from behind a wall, already at medium-close range, and we were left with the dilemma of whether to fire on you, or give you the first volley (which, at such a range, is quite devastating). It was kind of "run up to and prepare to obliterate each other on command." It was just awkward.
On an open map, that isn't so much an issue -- you usually form up at longer ranges, and so it is quite a bit safer. You have time to form up properly, and give orders. The battle isn't decided in the first few vollies, and therefore winds up being more interesting. There is room for maneuver, and the results are less "random" and more dependent upon use of terrain, good formation, and proper aim - insofar as compensation for range is concerned, over a long period of exchanged fire. You could even, in such a situation, implement proper tactics to advance on and engage an enemy.
Practically speaking, we shouldn't have fought in formation on the ruins map. However, had it been a normal game, even, I do not think we would have been at the same disadvantage fighting in formation on a wide open map, as we did.