I remember doing an experiment to see which unit provided the greatest bonus to number of arrows fired by a building... The result was, surprisingly enough, the Mongolian special unit (can't remember the name. Basically an enhanced horse archer).
A castle stacked to the brim with those guys produced a frightening number of arrows per round. I think it was something like 41 arrows each time... Enough to blanket a troop of infantry.
Yes, AoEII is an amazing game, and the expansion pack just makes things better. There are so many ways of making the game even more interesting than it already is, and it really is just a gem of a game to begin with. I grew up with the AoE series, and I still have some very fond memories from the first one.
AoEIII was okay, but it seemed like it was trying to cover too many different themes at once. Didn't quite have the same pizazz as the first ones. And the resource card system wasn't quite what it could have been... I did like the livestock, however. Not the most elegant methoed of handling them, but at least better than AoEII (although the sheep were cuter in the previous game).
As for future games, they seem to be moving up the timeline with each game (there's even a placard somewhere showing "all five" AoE games, with artistic renditions from each era marking their spot), and AoEIV has a modern soldier on it. Making modern-era games takes a different style than making games set in earlier ages, and I don't know how well they'd pull it off (if they do decide to go that route). AoEV was futuristic.