15 inches is too close. These will get 3 to 4 feet wide. I planted mine 3.5 feet apart to give them room to grow, and in three years they are almost touching. If you are impatient, put them a bit closer, but crowding doesn't do much for the health of a plant, IMO. I'd give them space to grow at the wall, too.

I don't fertilize newly planted trees or shrubs for the first year. You don't want them to put out a lot of new growth, you want them to put out a lot of new roots. Keep them watered when you don't get any rain, and don't let the root ball dry out completely.

I agree, plant them no closer than on 3 foot centers

There was a gap in the mixed border along one side of my yard, close to my neighbor's porch, and I wanted privacy so I planted some Emerald Green Arborvitaes along there. They were pretty big, six-eight feet tall, in 10 gallon pots. Like Saypoint, I planted them approx. 3.5 feet apart on center and about 4 feet from the lot line. This may look far apart if the trees are very small when you plant them, but it won't be once they're larger. When they are mature, I am hoping they touch and feather together a bit but don't crowd each other.

I also planted them in a sort of loose zig-zag pattern instead of straight in a row. I don't like straight lines and rigid geometric shapes in the yard, prefer a more naturalized appearance. I am also planting a mix of deciduous and evergreen shrubbery, and perennials in front of the Arborvitae, so essentially they are going to serve as a green backdrop.

If you plant a double row at 3.5 feet (or more) on center, with the trees in the front row planted between the ones in the back row, you'll get the quick privacy without crowding the trees. The 2 rows, of course, need to be, at a minimum, 3.5 feet apart. I guess this is just what Terrene did, I just think of it a little differently.

I've made the mistake of crowding trees in the past, and it is one I would recommend you try to skip.