BrickWood Box Q’s

I am getting started on my box and I was wondering if anyone has built one?
First question is how come we don’t use castable on the bottom slab? We us it on the top (grilling) slab but not the oven (lower slab) slab?

Anyone do the castable slab in place?
That’s what I did on my pizza oven slab. Worked good but didn’t know if anyone has done it on the firebox

Any good general info on what to look out for or things people wish they would have done different

Thanks KOB

Any thoughts on this?
Pouring my pad this coming week and then setting blocks