I have an area on a slight slope with solid rock about less than a foot down. I was thinking of building a retaining wall in one section to allow the soil to build up so I can plant fruit trees. Does it need to be watertight, or can I just pile a bunch of rocks and the soil will naturally plug up the gaps? Even just a foot or two would make a world of difference.
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Line the buried side of the wall with heavy landscape fabric.
It does NOT to be water tight, actually it is better if it has drainage. The original soil will be harder than soil you build up there, so water will run down two the wall on the original soil level, so if no drainage there it 1) may bring issue of standing water near the wall, and 2) create risk to your wall to just fell under pressure of heavy wet soil. To make sure your soil is not washed away with drainage use landscape fabric, it will let water through. If you have enough rocks. making rock wall may be an option, but you need flat rocks close in size. I use regular concrete blocks to build mine where I can. My property is slop in both directions, and also has a deep drop, where I have use terraces, so fixing rock retaining walls is my every day task. 
Concrete blocks need much less attention.
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Yes, I considered using CMU blocks, and even adding some mortar to hold it all together. Might require more work to make sure everything is level.
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I’ll see if I can’t find pictures from my original walls. A total of 3, each 8ft diameter stepped up the side of a hill. Each circle hold 4 trees. The smallest is no more than 1 ft tall with my tallest sitting about 3.5 ft above the ground.
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Couldn’t find any original pics on my iPhone but these would kind of help. I did all the work myself over the course of one summer.
Very nice! Much better than my crude efforts!
Very nice professional looking job Sean, that’s a real nice accent to your yard.
Love the look, gives me more ideas for future projects
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Thanks everyone! Just an FYI, this was all built using the (at the time) $.80 block from my local Menards. Nothing fancy, didn’t go with the $5-6 block, etc. Everything I read, every video I watched said to bury 1 whole block and then 1" for every foot of height. I threw that right out the window. For me the blocks were around 6" in height and i dug down about 1 to almost 2 ft in most areas. I figured if it was just holding back some dirt those recommendations would be fine but I have seen Tree roots bust right through concrete walls, upheave sidewalks, etc. so I said the more weight I can place at the base the better. Also after the first course (which is resting on 6" of paver base and sand) I used retaining wall ahedsive on EVERY course including the cap. Again, everything I read stated no need until the last course and the cap but I figured for $4 a tube (the large tubes) it was worth it. I even had my real tine tiller in there that got away from me at one point. Thing ran straight into the wall and didn’t even cause it to move. It’s going on 5 years right now and I just checked it last year for level and found it hasn’t moved at all. I must have done something right. Lol!!
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That’s impressive! Especially when considering the freeze/thaw cycles in your climate.