First Time Bricklayer Needs Advice on a Garden Retaining Wall

[Grand chippy]

I'll keep it short.

My garden is 4 x 8 and on a slope of 30 degrees made from clay.

How deep does the footing need to be from ground level? 225mm? Or do I need to make sure the first block is below ground level to tie it into the ground?

Because I will have about 4m of garden behind the wall should I use a single skin of hollow block or a double skin of a 100mm dense block? Or more?!

Should I use rebar?

Should I tie the feature wall in with the retaining wall?

When I put in the perforated pipe on top of aggregate behind the finished wall, where do the ends go as I'm in the middle of a terrace?

Complete rookie I know but I'd rather give the project a bash myself then pay a fortune for someone else to do it!

[welsh brickie]

if its on a slope its best if you step the foundation, the depth of the foundation depends on ground conditions, the foundation needs to be dug at a minimum of 300mm deep with a minimum of 150mm thick concrete, also the thickness of the wall will depend on the height, if your building it a metre high with soil against it, it will need to be 215mm thick,

[Grand chippy]

Thanks welsh brickie.

Do you reckon the hollow 7.3n's are better than the dense and then filled with concrete and rebar? Or just stick to the sense 215mm blocks?