Yes, the series is an RPG/RTS hybrid.
The first game Order of Dawn came out in 2003. In the aftermath of the Convocation (big, magic ritual through which individual mages of the Circle tried to attain unparalleled power) the world was torn asunder, with people stranded on isolated floating islands. The only surviving Circle mage, Rohen, tries to fix things, building portals to connect the islands etc. In his efforts he gives one of his rune warriors possession of their rune. Rune Warriors were soldiers in the war between mages, their soul bound to runes, giving the bearer of a rune control over the person. The player was that Rune Warrior. You leveled up RPG wise, combat or magic, and stats that govern effectiveness of ability and equipment you could use. You also had a rune board, which allowed you to control your own army of rune bound souls. Some strong souls would become your party members, and the races you could lead in the RTS part were also represented as runes.
For the RPG part you had a proper adventuring party of rune warriors. You could also zoom into over the shoulder third person, which was necessary for a handful of quests were you needed to find hidden switches for example. RTS was normal RTS with base building, upgrading units etc. The RTS part could get a bit weak when your party was strong enough, or on maps that started in "RPG Mode" until a trigger turned on the RTS part, allowing you to clear the map before enemies spawned units - of course you had to actually invest in the game to figure that out
PC Gamer UK hated the game and found the story confusing, boring or plain bad. I found the person who wrote that article to be an idiot
Spellforce 1 had two stand alone expansions.
Breath of Winter had you playing a different Rune Warrior, also freed. The story was more mainstream fantasy, catering to the people who had not understood the first game. You tried to find the queen of the elves, who alone with her song could put an ancient dragon back to sleep. Gameplay wise it didn't really differ from the first game, but had new opponents and units. And it had the first non-human party members.
Shadow of the Phoenix allowed you to import a previous character, either from Order of Dawn or Breath of Winter, and tied the stories together.
For co-op skirmish heroes the game included an extremely high level quest giver who was a teaser for Spellforce 2.
Spellforce 2 was a bit of a spin off, the concept of Rune Warriors abandoned for the Shaikan, a human tribe that had entered a pact with an ancient dragon and gained power through its blood, pledging servitude in return. That said, while it changes who the player is, where their powers come from, it still behaved more or less like the original. I keep meaning to finish it and its three expansions, but it came out at the wrong time for me to play it then.
Spellforce 3 is the recent game. It is a prequel, playing in a time before the Circle of Mages even existed. You don't need to have played the other games, but it ties directly into the original game and its expansions, so people who have played those will, from a point in the game onward, have a more rewarding experience.
Fun fact: when I made a dual wielding rogue in Dragon Age, I was basically returning to my original Spellforce character.