As per Veins, there are six routes.

Anyone can, without rolling, just do 1, 2 or 3 with a rope and 2 without a rope. Outside of combat or other time-sensitive, stressful situation. If there is fighting or other stress, then the dice come out.

Anyone can try to roll on routes 3 and 4 without a rope.

Those who have Second-Story Work can try to roll on routes 5 and 6 without a rope. They also have advantage on routes 1 to 4.

Regardless, no-one can be encumbered when trying route 3 or higher.

Use pitons, hammers etc to set up a rope for your friends.

Climbing DCs are based on how much time you take to look at the route before you start climbing, not on how difficult the route itself is. That's why you need light, you can only study what you can see. You need to reroll once you reach new, unstudied walls.

However, if you do fail, the more difficult routes have more dangerous consequences. And also, routes 5 and 6 you can't even try, they're impossible to normal people. (Spider Climbing spell is great, or being a monster with Climbing.)

Climbing rolls are

• Athletics when you try to climb caves, houses etc
• "Athlobatics" when you try to climb on the backs of giants, monsters etc. They use Acrobatics to shake you off.
• "Athlobatics" when the giants, monsters etc try to scrape you off, knock you off etc. They use Athletics to do so.

[Or vice versa if someone small is climbing on you.]