Which brings me to what I think is the primary reason that this game is truly superb. You're not just destroying your opponent's armies, you're slowly building your own. Each unit must be carefully nurtured and protected, so that it can advance and upgrade into newer, more powerful types of units. In many strategy games, individual units are just a resource to be spent in order to achieve the objectives. If some are destroyed, it is no great loss so long as the battle is won. In this game, each unit matters. Having an experienced unit destroyed can seriously weaken your entire army in future battles, and can be very expensive to replace.

Furthermore the vast selection of unit types allows players to customize their army mix to try different strategies, further increasing the replay value.

Balancing this need to protect your units and keep them at peak capacity is the need to advance and capture objectives quickly. This will make for many agonizing, frustrating, and/or heartbreaking decisions throughout the campaign.

Ultimately this is one of the first, and best, strategy games that realized that the ability to customize and grow an army over time was far more interesting in the long term than simply erasing enemy units off a map.