Mozie wrote:

1. I don't intend to pull up the bricks (underneath the bed itself) but
will ensure there is sufficient drainage. The passage is on a slight
tilt/angle so shouldn't be a problem. Anyway, there is also only
building rubble under the bricks, so there seems no point.

Try pouring some water on the patio and see if it drains. If so,
you're probably fine.

2. I'm wondering if I need to seal the bottom and inner sides.

I don't see that there'd be any advantage in sealing it.


3. I intend to put a layer of stones in the bottom, also to aid
drainage.

Sounds good to me.

4. There will be 3 or 4 sections separated also with a brick wall to
add strength to the structure and also to be able to rotate the crops.

You might like to look up keywords 'square foot gardening' for some
ideas on layout. I suggest you put a trellis behind the whole thing,
on which you can grow runner beans (which are both decorative and
edible), 'winter' squash, melons (!) (use old pantyhose slings for
support as the melons near ripeness to keep them from falling to the
ground when ripe), and cucumbers in the summer, or peas in the
spring/fall/winter depending on your climate.


5. I'm not sure what the minimum depth should be, especially if I
intend to grow carrots and onions as-well.

If you grow only round varieties of carrot, onions, and radishes, you
can get by with 6 inches of depth. With a foot of depth you can grow
almost any kind of carrot and some kinds of turnip. But if you make it
18 inches deep and put a foot-wide rim along the top, you have not only
a planter-box but a bench--dual use of the space. Plus cruciferous
veggies have a tap-root (brocolli, mustard) that would probably enjoy
the extra depth. The taller height is also less likely to be 'missed'
and tripped over by careless walkers.

Is there enough room that 18 inches of width is possible? That would
allow you to grow some more space-hungry crops like tomatoes or some of
the larger brocolli and cauliflower varieties. You can still grow
these in only a foot of width though, they just might turn out smaller
and yield slightly less. As an alternative you could grow these big
guys in 18 inch pots scattered around the patio--that way you can move
them if you wish, but also because tomatoes (and potatoes) tend to grow
fungus species that can ruin the soil for other kinds of plants
(especially strawberries).