Been thinking about this for a while now and have finally decided to take on this project. Gathered my fire bricks first (43% alumina but at the right price). Picked up a mixer and brick saw second hand pretty cheap and hope to sell them when I'm done and get most of my money back. Next decided on a spot and that just happened to be where a golden cane palm was. 3 days digging later, the palm is now gone. No mucking around and boxed and poured the foundation slab, starter bars in place. Again, no mucking around and a week later, I have my blocks layed, core filled, and all my steel reinforcing in place, ready for my top slab. I think a tank would bounce off this thing. Now I have a question about the insulation layer above the top slab and beneath the floor tiles. Will it hurt to recess this to be level with the slab height. I'm trying to achieve as little a step up into the oven as I can but am concerned about it filling up with water if it rains. I was planning to recess the insulating bricks into the slab or build up a second floor layer around it. Just concerned about it becoming a swimming pool when it rains. Thanks for reading.