Ambitious but amazingly successful, it's obviously difficult to improve and outdo because it almost spells perfection.
Age of Empires II: The Age of Kings is proves to be very difficult to improve. It's one of those few games which can exist forever independently and need not be improved or developed into some other dull succeeding franchise.
Although AOE2 did not quite break huge sales records for RTS games, it gave the world something from which one can easily understand global history and different cultures of man. With its highly addictive gameplay, it's a fun way of learning exciting things which usually are perceived to be boring (since the medium always had been tedious and uninteresting).
Believe it or not, AOE2 can--in a way--help you win chess games! Improving analysis and wise decision-making skills are just two benefits that gamers can get aside from pure entertainment. So, it's more than just worth the money. That's a relief.