Another great thing which is in both games is that you don't build just one soldier at a time, but a hord, or a battalion. You can get a hord of 15 orcs in the first game for free. But they are the weakest. Just as weak as goblins, though the latter can get upgrades, and have poisoned blades. Now, in thee first game are four factions: Rohan, Gondor, Mordor and Isengard. You cannot fight in fortresses except in the campaign, which follows the book and the films.
The second game has six factions: Men, Elves, Dwarves, Isengard, Mordor and Goblins. In the expansion of the second game, you have a seventh faction: Angmar, which is two thousand years before the War of the Ring.
Now for the campaigns. As I've already said, the campaigns (good and evil) of the first game, follow the story of the book and the films. Not exactly for the evil campaign because you destroy Edoras, Helm's Deep and Minas Tirith.
In the second game, the two campaigns (again good and evil) are about the War in the North, being the dwarves and the elves fighting mostly the goblins, but sometimes Mordor, and the other way round. The third mission for evil is the goblins fighting Isengard for the Shire.
The campaign in the Rise of the Witch-King is different, because it happens thousands of years before the War of the Ring. In this campaign you are Angmar, trying to take over Arnor. When you have finished this campaign, you can play this thing called"Epilogue", which is some giant battles between Angmar and Men.
In all three games, each faction has some magical powers, but to unlock them, you need some kind of points. You start with enough to by one of the first powers, but to get more points for the more powerful powers, you need to kill or destroy your enemy's soldiers and buildings. These powers go by steps, the weak first, all the way to the most powerful.
In these games, you can also get heroes (examples, Aragorn, Legolas, Gimli, Lurtz, Saruman, ...). The heroes also have some powers, but they only need to go up a level to unlock them. These heroes are individual, cost a lot of money, but are very powerful and resilient. When a power is used, it must recharge before being reused. The stronger the power, the longer it takes for it to recharge.