(I edited your post slightly because the link was breaking the template.
Years ago, when I first began gardening, I used to add rocks/broken crockery to containers in order to improve drainage. But after many years, I discovered that it didn't do much good, and in fact, often caused damage to the plants' roots. IMO, the only
advantage to adding rocks and such to a container is to keep a tall plant from becoming top heavy and toppling over, or to keep the container from being blown over by strong winds. Your garden isn't likely to have either of those problems, so I would advise against adding a layer of rock. If I were going to add anything
under the soil in a garden such as yours, it would be a layer of activated charcoal, to keep the lowest level of the soil from turning sour. That could be quite expensive, though, and is not something I would consider a requirement.
One of the problems I found with crockery or rocks in the bottom of a container is that the plant's roots grow down into that layer. Then, when you pull or dig up the plant at harvest (or to repot), those rocks come up along with the roots. They become mixed into the soil, where they do no good at all, and can interfere with root crops such as carrots. (Gardeners don't usually prefer rocky ground for their gardens.) Short of removing all the soil from the container and sifting out the rocks, you end up with no advantage at all. JMO, though. (As you can see, I am not
a proponent of rocks or crockery in the bottoms of containers!
Regardless of whether or not you choose to add rocks, and whether you do is
entirely your choice, the only "layer" I would recommend would be a layer of mulch on the top to prevent weed growth. That should be about 3 or 4 inches deep.
Consider that, if you allocate 4 or 5 inches of the 18" depth to a layer of rock in the bottom, then add 3 or 4 inches of mulch on the top, your plants are going to end up with less than a foot of soil in which to grow. That makes for a really shallow garden, and not much room for the plants' roots. Whether that matters will depend solely on the plants you wish to grow. If you plan to grow greens and herbs, that's plenty of soil depth. If you were intending to grow tomatoes, though, it might not be enough. In general, a tomato plant requires about 14 inches of soil depth, at a minimum.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams