How to get through Karthspire?

Skyrim will not prevent you from going to an area that is too high level for you.

Skyrim's Level scaling can put in you a dungeon that kills you very quickly, even in one hit. This means that you need to come back later. The most obvious example are the giants and mammoths that roam around. They WILL kill you in one hit in all but the lowest difficulty.

Luckily, the Forsworn camps are open and can be easily departed from. If you ever encounter a cave that has a name with anything to do with eyesight, make a manual save and prepare for the worst, because those caves have a one-way gate and you cannot easily leave, and they have some of the more difficult enemies in the game.

If you desperately want to get through that area, you can always change your difficulty to the lowest setting and steamroll over all the enemies.