Perhaps you can do like the pros and install below grade PVC conduit in and to the general areas where you anticipate needing power. Put a nice sturdy pull string or wire (electric fence wire is relatively cheap and comes in long rolls) in the conduit to use when you eventually need to pull the electrical conductors for the landscape lighting. Use pre-curved fittings at corners, both horizontal & vertical, but do not accumulate more than about 180 degrees of total bending (horizontal and vertical combined) between open access points in the conduit system where the pull string/wire can accessed. Because of friction on the inside of the conduit, the bends make pulling the electrical conductors more difficult and too many of them in a run will make it impossible to pull the electric wires through.

Rmorelock is right on. The only other thing I'd add is to use 12/2 wire that is intended for underground bury and good waterproof connectors with silicone.

My question is: How are you intending to illuminate the wall? From below with wash lights or from the top of the wall down using integrated hardscape lights? These 2 options will require you to run your cable to different areas (both pretty easy from what I see in the photos). It is not necessary to place 12/2 cable in conduit. It is intended for direct burial. The only reason to ever pre run conduit is if you are going to be locking yourself out of an area with a solid surface such as pavers or concrete or if you are intending to place lighting within a solid surface. For your application, you can easily dig in your cable at any point (preferably when the block wall is up and you have all of your plantings in place. If you are intending to place hardscape lights in the wall, I would suggest not gluing down your capstones until after the lights have been put in place. If you have already glued them, it's still not a big deal. They can be popped off as needed and re-glued when the fixtures have been affixed in place.

Lite4 is correct. No need to run conduit, just run 12/2 cable and you'll be fine. If you can plan for where you want the fixtures it will help, leave a little slack in those areas for easier installation. It makes a big difference where you plan on installing the fixtures. Because the wall tapers outward under the cap many fixtures may not be able to illuminate all the way down the wall and you may end up with hot spots of light at the top so be careful of your fixture selection.