I have always used mine on grass, and it is quite sturdy there. Each run is put together of three separate pieces. The two long sides are made of foldable panels. Then there is one piece for one of the short sides. And then the foldable lid, which forms the roof and the other short side. The pieces are held together by metal rods, that you have to push into the ground, and the roof is attached with several ......hmmm.....how do you explain that in English.......overlapping pieces of metal.
If you are not going to move the run much, then you can use cable ties and other strong things like wire to keept the pieces together. And you can do the same, if you want to use it on concrete. You can get those little shelters and attach them to one of the small doors on the sides, but I would rather put the shelter inside the run, unless I was watching the bunnies all day. I am not sure if the shelter could be pulled away by a fox or not.
The run is quite heavy, as it is made from metal, so I don't think a fox would be able to lift it. My bunnies don't dig, so I can use it on grass, but I don't put them in the run, when I am not around.
When I put two runs together, I usuall let the two roof pieces overlap in the middle, and then I can fold that bit open and put bigger cardboard boxes and tunnels etc. into the run, and also catch the bunnies more easily than while waiting at one end. :lol:
I can take close-up photos of some pieces, if someone wants to see something in particular.