2. Ascending and descending vertical surfaces is the ability of
the thief to climb up and down walls. It assumes that the
surface is coarse and offers ledges and cracks for toe and hand
holds.

Climbing walls is attempted whenever needed && desired.

It is assumed that the thief is successful until the mid point of the climb.
At that point the dice are rolled to determine continued success.
A score in excess of the adjusted base chance indicates the thief has slipped and fallen.
(Your referee will inform you of what amount of damage has been done from the fall.)
Success indicates that safe ascent or descent has been accomplished.

Note that in some cases a third d10 will have to be rolled to determine the success or failure. (WSG)

DMG:

Climbing Walls: <or: DSG+WSG>

This is probably the most abused thief function,
although hiding in shadows vies for the distinction.
The ability to climb walls is something which is acquired through training && practice,
just as are most of the other functions of the thief.
The rate at which vertical or horizontal movement is possible depends upon the texture and other conditions of the surface.

WALL CLIMBING TABLE, FEET PER ROUND OF CLIMBING

                                                                                            Condition of Surface *

Wall Surface is Best Described As:
Non-Slipppery
Slightly Slippery
Slippery
very smooth -- few cracks
6'
3'
0'
smooth but cracked --- somewhat rough
12'
6'
3'
fairly rough and some cracks -- very rough
18'
9' **
6'
rough and with ledges or many projections
24'
12'
9'

* SLIGHTLY SLIPPERY surfaces DOUBLE chances of slipping and falling.
SLIPPERY surfaces make chances of slipping and falling TEN TIMES more likely.
Thus, a slippery surface cannot be attempted successfully by any thief under 6th level,
and even a 10th level thief has a 10% chance per round of slipping and falling.

Be sure to check each round of vertical || horizontal movement for chance of slipping and falling.
Surfaces which are inclined inwards move towards greater degrees of difficulty --
a non-slippery one being treated as slightly slippery,
and a slippery one being virtually unclimbable.
Surfaces inclined away from the perpendicular on an outward angle may be treated as either a better surface condition or
a rougher texture, if the degree of incline is sufficient to make climbing easier.

** Most dungeon walls will fall into the fairly rough to rough category.
Some will be non-slippery, but most will be slightly slippery due to dampness and slime growth.