That is so unusual how the roof is made. I bet it will stand up to just about any weather condition with all that concrete and rebar (steel rods). It will be a fortress!!! Your own little castle! Is that how most of the roofs are built in Belgium? Built to last a long, long time! I wonder if it keeps the homes cooler in the heat of summer and warmer in Winter?
I love, love, love all your raised beds throughout the various levels! That really looks wonderful!! Have you considered growing some plants from seed? I grow a LOT of daylilies! Almost all from seed. I have over 300 daylilies. Not sure how such a plant would grow in Belgium, but they are beautiful plants. So many plants can be grown from seed. A lot of folks here on ATP do Winter Sowing in covered containers in late Winter/Early Spring and then plant them into their beds when Spring has arrived. Many plants get big quickly .... but you have to remember the old saying about growing from seed .... first year sleeps (roots are developing more than foliage), second year creeps (you start seeing more above ground growth), and third year leaps (you see a burst in growth and start seeing a full grown plant)! I garden on a budget and I grow a lot of my plants from seeds or cuttings. I have learned the true meaning of "patience".
My avatar is a hybrid daylily grown from seed that I will probably name and register in the future. But there are so many other plants you could grow as well! The options are endless, in my humble opinion.
