http://www.moddb.com/games/the-protectorsDon't need Warlords Battlecry 3 to play it. You can just download straight from MODDB and play. Be sure to get the hot fix.
The Protectors is pretty much how I'd think Warlords Battlecry 4 would be like if it was made today. Except maybe not as advanced graphics.
For those who don't know what Warlords Battlecry games are like.
Think Warcraft 3, but you get a persistent hero (the only strategy game series to ever have one, still to this day there are none as far as I know) that levels up, gets spells, skills and what not. There are many races to choose from, Daemons, Undead, Barbarians, Fey, Dwarves...the basic fantasy stuff. There are also more unique ones like an insect based race, a dinosaur based one, and there is a weird one with floating eyes and stuff.
Warlords Battlecry 2 had a risk-style (very small) game map to conquer. Warlords Battlecry 3 is more a storyline to follow, which is how the 1st one was. WB3 has a nice sized map, and offers a lot to do.
The Protectors does a lot of stuff, and a lot of the features you can see on the page. But, it adds its own storyline, and allows players to create their own storyline for the standalone mod. It also improves AI, new items, new editor and a bunch of stuff. I don't want to write a huge novel here, but again, you can see on their page.
As an additional note, I'm not a team member to it. More a fan. And its not very well known, and Warlords Battlecry series itself is kinda an unknown game series that not that many people know of. I found it to be one of my favorite RTS and miss a lot of features in it that modern strategies don't have (like the persistent hero).
Also, as a note. Like Warlords Battlecry 3, The Protectors isn't very balanced. Warlord Battlecry series itself is incredibly unbalanced. They focused on fun and just to be entertained by it. I never do multiplayer though (which in games focused on multiplayer, balance of course is more important), but singleplayer is probably even to this day, still some of the most fun I have in an RTS game. Usually I'm more of a 4x guy, but WB3 is just pure fun. The Protectors adds and enhances a lot of that, and improves a lot of things.
And again, don't need to buy anything or pay anything. Just download, unzip and play