I have a rock retaining wall that has seen better days. Some day we may replace it, but for now, I would rather try to do some patching to make it more water right and also to fasten the top stones down so they aren't a hazard. It seems like it's meant to be fairly "primitive" looking, so I'm not super concerned that I'll mess up the look of it. I'm more interested in ensuring that I use the right materials & tools to do the work so that it will hold together as long as possible. What type of mortar do we need? Are there any tips or tricks I should know? Thanks!
Your “wall” looks more like a “curb” than a wall. I can’t tell from the picture, but it doesn’t look like it’s holding back any dirt (like a retaining wall).
I’d use Type S mortar because it is waterproof (good for exterior use) and has a high strength value. (Type N has a medium strength value.)
When installing the mortar, you don’t need to have a “full bed” to lay the stones into. Because it’s not a retaining wall and because it’s not tall, it doesn’t need a carefully laid depth or consistency…perfect for a DIYer. Just match the stones as close as possible and install the mortar in a pattern that matches the existing mortar joints.