Looking at the Civilopedia description:
Exactly what is a "stealth attack barrier" (circled in red)?
Stealth attack work as it was explained by Elephantium and LKendter. Here is the explanation in the civilopedia:
In CCM2 many hundreds of units were set to have partly hundreds of units for stealth attack. These units have descriptions in the civilopedia. The settings in the editor for stealth attack were an immense amount of work.
Stealth attack barrier is a border, behind that stealth attack isn´t working. Per example your archers can attack a redlined unit and eleminate it, even when a veteran warrior as a defender is in the same tile, as there are at least two stealth attack targets for the archer in that tile. It wouldn´t work with a spearman as the defending unit, as the spearman isn´t a stealth attack target for the archer. These options become especially interesting when bombards, cannons and their upgrades appear on the battlefield.
When a city has a stealth attack barrier (=walls), redlined units inside that city have a better chance to survive, as stealth attacks here are not working and the redlined unit can recover.
I do not understand how Enslavers could attack on Turn 7.
It was turn 7 of the part of ThERat. He took the game in turn 61, so his turn 7 was turn 67 of the game - and here it is abolutely normal, that enslavers are in the game.
I downloaded the only save file of the game in turn 60 and also managed to kill the second enslaver near Algier without any problems. The clue, if you have no proper defenders, is to handel the workers and warriors offensively against enslavers. The workers (starting with one) should move near to the city to search for enslavers or holy men. When an enslaver is detected, you normally need two warriors to kill the enslaver; one for redlining him (when he kills the warrior or is forced to retreat and one for moving on the new built road and taking out the redlined enslaver. Your change to government City State also favours this tactics, as you have a worker ability of 100 % and three workers near Algiers, meaning they can build a road each turn to pursue the enslaver with your warriors. An enslaver has next to no chance against such a combo. The roads used for pursuing the enslaver additionally increase the output of your city (in that case Algiers). Of course, if you don´t hunt the enslaver and kill him, he can recover and soon you face the attacks of two enslavers (the recovered one and a new one).