I have a habit of looking back to 3.5 rules when 4th edition has left something ambiguous.
According to d20srd, climbing a "rough surface" like a brick wall is a DC 25, but having an opposite wall to brace against gives a -10 DC, for a total of 15 Athletics.
Someone in the party should be able to make this easily, after which they can let down a rope. At that point, I wouldn't even require a roll.
