The Brew Cabin

Hi, I currently have a cedar deck in my backyard that I would like to replace with slate. However, I want to keep the same height of the deck which is about 2' from the ground. I'm planning on building a retaining wall out of Allan Blocks.
Firstly, do I need to tie in my blocks with clips or ties?
Second, do I need to lay down a concrete subbase before filling in the patio area to bring it up to the required height? Should I allow for drainage?
What should I use as fill?
How much slope will I need for 20'x30' surface area?
I'm thinking of using a random slab pattern but I am uncertain whether to use real slate or to go with a stamped concrete finish. Can you give me some pros and cons for each application.
Thank you
rhammer13

I use Allan Block for my retaining wall projects. Where about are you located? Have you read the Allan Block instructions? They go into great detail.

Depending on your site situation you might want to consider using the Geogrid to tie the wall into the patio area. I would not use concrete. I would build up 6" (150cm) lifts of compacted ( with a vibratory plate compactor) CR6 ( US term). Installed properly the Allan Block has built in drainage. But again depending on your site. I run a 4" (100 cm) perforated pipe to daylight with some fall inside the wall at the base.
I would be partial to the real slate.
LM