In reply to Thomas_Climb:
The wall is probably not concrete. It has been rendered (covered) with something usually called pebble-dash which is probably not more than 1cm thick. Below this there is probably brick or cement blocks depending on the age of the house.
I'd be fairly sure that the underlying brick/block wall will be strong enough for you purpose and you should probably start by fixing a frame made of 50mm * 50mm timber (approx size) using "frame fixings" (basically very large screws with wall plugs already attached). Frame fixing are the things used to hold doors and windows in place and, if placed correctly, can take the load. They are designed so that you can drill the same size hole in the frame and the wall (which makes things much easier because you can drill the holes in place to get them perfectly aligned). If you use a long big enough fixing you can go straight through the plywood and frame at the same time.
eg https://www.screwfix.com/p/easyfix-frame-fixings-8-x-120mm-10-pack/17480
Tip: have the correct size screw driver bit (usually PZ3) - it makes things much easier and you can bang them in with a hammer most of the way - just make sure the plastic plug is all the way in first.
The chances are you'll want to make you "wall" have a fixed over hang or variable over hang in which case you'll best have one frame for the wall and one for the board.
The frame has with the structure and strength but also, crucially, provides a stand-off from the wall giving space for your bolts and screws (from the holds) to stick out the back without hitting the wall. If you want to bolt holds onto your panel you need "T-nuts" (also Tee nuts) and most climbing holds take M10 bolts. Use a screw (or something else) to secure the t-nuts at the back because the has an annoying habit of falling out if you don't.
Use exterior treated wood and exterior ply and paint the ply (fence paint) or it will get damaged by the weather much faster than you want. Ideally paint all sides and edges before you put it up.
It you are going to an variable overhang make a hinge at the bottom between frames using M10 of M12 bolts (easier to use threaded bar) and use lengths of chain at the top to set the angle. You usually use a bigger chain than you need for strength so you can get bolts in the links for fixing and adjustment.
Post edited at 15:52
