Discovered it accidentally. The strategy guide doesn't mention it and a google search doesn't bring it up either. I'm curious if there are others like this.
(Spoilers on how to "unlock" it below):
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In level 6, the extremely difficult one where you have to fortify the Keanor city across the river as well as defend against the enemy on your shore, you can let Keanor fall, in which case Sartor will be captured. The next "secret" level then will be of you rescuing Sartor, after which the game will proceed as normal. Pretty nice.

Ha, nice discovery! I played that mission a couple days ago, it's quite intense. I quite enjoyed this map, reminded me of some levels in Stronghold Crusader where you can send your troops onto the walls of your allies castles and reinforce their defenses with extra towers and stuff.
Tzar is quite fun, if only the pathfinding AI wasn't so abysmal. And the mission after this one is a pretty lame attempt at Commandos, quite aggravating that there's no way to know how far spies can see, sometimes you can walk right past them and other times you get caught for no apparent reason, fortunately it's a short mission.

Very true. It's especially crap when you have to cross bridges. I just finished level 9 (the one with an island between you and the enemy, connected by 2 bridges on both sides), and it was a nightmare trying to get my guys across.

Yep, it's almost ragequit worthy. Troops simply can't figure out how to get in line properly, especially siege machines. Even if the space is wide enough they either won't budge or will try to go some random other way or get stuck. Two of my ballistae got stuck and blocked the entire bridge in that level (I blew up all but one bridge), and I couldn't move those units forward or backward. In terms of pathfinding AI I've never seen anything as abysmal. In the end I had to precisely guide one of the ballistae back and then the other one, ended up loading them onto ships which is a hassle but less of a hassle than the bridge. The bridge also made some of my units disappear, not kidding. Just vanished into thin air..