Plywood panels are definitely the preferred wall material but they need some sort of framework to provide rigidity and to link up several panels to create an extensive wall. Usually you build a framework first and then attatch plywood or OSB so the framework defines the shape of the wall. I say usually because you can build a wall in the opposite order. I've tacked up plywood pieces in a complex shape and later went back and added framework behind the plywood.
The basic framework to which plywood panels are attatched is simular to that used to build a house. For basic walls, wood studs are oriented with the small dimension against the plywood and spaced every 16 or 24 inches apart. House construction typically uses 8' long studs every 16 inches with additional 2x4's joining these together at the ends (called plates). For taller walls (like the second story of a house) a second 8' tall second of studs joined by plates is placed on top of the first. This standard method of construction works ok for vertical walls where the forces are straight down in compression. This construction method also enables portions of the framework to be prefabricated in easy to move sections on the ground. These two attributes are the reasons that houses are built using this method. However, climbing walls are not houses. For an overhanging climbing wall you will end up with forces that are not just straight down against the spine of the wall in compression. The weight of a climber will end up pulling against the middle of a wall (shear force) so you want to use 2x4's or preferably 2x6's that are as long as possible - not broken up into sections seperated by plates, a method that while strong in compression is weak in shear.

Hopefully this cavet will serve to demonstrate that climbing walls are a bit more complicated structurally than houses. And as the overall shape of a climbing wall becomes more and more complex, more skill is required to build the thing


Walls can be grouped into 4 levels of complexity. Basic walls have vertical sections, simple overhanging walls, ramps and roofs where most panels meet at 90 degree angles. These walls are easy to construct because the frameworks tend to be simple and straightforward. They are also relatively inexpensive because they use a minimum of framework materials, and don't waste much plywood because few cuts ae made. It is also relatively easy to asertain the strength of a simple framework. If necessary, an archeticural engneer could sit down and produce blueprints of such a wall and certify it structurally sound if this becomes necesssary because of liability concerns. The author will now make up an arbitrary climbing wall complexity scale (the CW scale) and assign this type of wall a rating of CW-1. Just about anyone who is handy with a hammer and can saw straight can build this type of wall. Someone who has worked as a carpenter can build such a wall without even needing to think much.
At the next level of complexity(CW-2) plywood sheets are cut up into more complicated sections (typically triangles) and fitted together to form a more elaborate climbing surface. On a pure CW-2 wall you will have few if any 4x8 sections of flat wall, and few if any 90 degree angles. Most commercially constructed rock gyms fit into this classification. The best way to envision this type of construction is to consider the shape of the F-117 Stealth Fighter. This wierd flying machine consists of lots of triangluar flat sections fitted together to form a more complicated overall shape. These type of walls tend to waste a fair amount of plywood, and require considerably more skill in their manufacture because framework studs will end up needing compound miter cuts. A CW-2 wall with a wood frame will require someone skilled in carpentry to spend considerable brain power to figure out how to make it all fit together.
Most commercial wall companies use a welded steel framework.to build CW-2's. It is a relatively straightforward process to weld steel bar stock together into triangular or square sections, measure the shape of these sections, cut out a piece of plywood and bolt it to the steel frame.
At the next level of complexity (CW-3) the wall no longer has any "flat" sections. Everything is curved Preferentlally all the curves are convex because this results in a more difficult climbing surface and it won't break modular holds like a concave surface will. There are two ways to make a curved wall. The most straightforward is to build a rebar and wire mesh structure and spray on concrete. Many zoos and high level mini-golf courses have these artificial rock structures.
You can also make curved surfaces using wood.


With and a lot of thought needs to
Illustrations


FRAMEWORK BASICS

There are really 2 types of framework. Sheathing framework, which is directly attatched to your plywood, and structural framework, which holds up the sheathing framework. The job of the sheathing framework is to hold together all the plywood panels to form one continuos rigid surface.

illustration, both types of frame and

For your sheathing framework, you use 2x4's or 2x6's, every 16 or 24 inches apart, oriented with the small dimension against the plywood surface. The choice between 2x4 or 2x6's is determined by the type of wall. In general, long sections of overhanging wall should use 2x6's (or 2x8's). Vertical walls can usually get by with 2x4's.

Attaching panels to frameworks
Screws
The easiest method to attach plywood panels is to use 2-1/2 or 3 inch long drywall or deck screws. These are inexpensive, have tremendous holding power, are easy to place with a cordless drill using a Phillips head bit, and you don’t need to drill pilot holes as with wood screws. Because drywall screws are hardened (regular wood screws are not), the screw heads do not strip easily so you can remove them fairly easily if you want to take your panels down.
The number of screws to use per panel depends on the gage. The thinnest screws are 6 gage and tend to be somewhat brittle. If you use these you should use alot - about one every 4 inches. I don't recommend you use 6 gage screws on an outdoor wall since they can snap under the stresses of the wind. Eight gage screws are considerably stronger - although they require a more powerful drill to place them. With 8 gage screws you can place them about 8 inches apart. Ten gage screws are very strong but also very difficult to place and are somewhat overkill for attatching panels.
One important consideration for your screws is the amount of thread on the screw. You will have the best results with screws that don't have threads that go all the way to the screw head. The ideal screw for attatching panels has three quarters of an inch of unthreaded section below the head. This will allow you to use the screws to draw your panels up against the studs. Screws that have threads all the way up to the head stop turning as soon as the head is flush with the surface.

Nails
Nails also work for attatching panels although they can make it extremely difficult to take a panel down if you need to add or replace T-nuts at a later date. If this isn't a concern, it's hard to beat a nail gun for quickly attatching panels. Nail guns are somewhat expensive although you can easily rent them. This could save you money if you are also renting a lift to attatch your panels on a large wall. For panels on overhanging walls and roofs, you should use at least a few screws to keep the panels from loosening up over time.
It might be useful to use both nails and screws to attatch panels. Sometimes it's easy to quickly sink a few nails to tack a panel into place and then use screws to securely fasten it.

Bolts