One point of reference I'm going by is a scenario game I played a few weeks ago as Spain in an American colonies game (North and South America map, starts at 1600 with all the major colonial leaders and natives like Monty, etc.) I had South America all mopped up and had black-dirted Monty, postponed the take-down of the Mayans (pretty much the way Spain did it in history), and was recovering the economy, trying to squeeze out of the Strike Zone with all the expansive land mass. During that time the Navajo (Genghis Khan) built up a stack comprised of about six crossbows, six pikes, 10 to 20 keshliks, and easily 30 to 40 cats. It was just cats for daaaaaaaaaaaaaaaaaaaaaaaaays, and I knew if I just hunkered in my border city waiting with the skeleton crew of 2 musketeers and a conquistador, they would be toast on a stick.
Fortunately I had Steel and had just finished Iron Works in a high-prod, high-food Tenochtitlan, and so was then able to crank out 1 grenadier per turn. Goditso (Genghis) was aiming for the one tile approaching my city that would not attack from across the river, a plains tile to the south (which meant he had to march his stack around some impassable mountains and along 7 or 8 plains and desert hills to get there). So I cranked out grens with Guerilla II, layering the defense on each tile in his path. Each gren was suicided, but they went down Thermopylae style, taking crippling numbers of Navajo with them. By the time the Navajo were at that first Mexican border city, they had about 10 cats, 2 crossbows, and about 4 keshliks, all severely wounded. Then I switched to conquistadors in the IW city and mopped up. The musketeers never had to fight as the Navajo still had to sit and bombard and try to heal a bit.
Across flat land the defense wouldn't have been as easy, but in an all-horse counteroffense I think I would have suicided more units (at least at first).