I recently received this cubby structure second hand and have since removed the roof as it was falling apart. The second image is what I now have left, the slide is detached as I plan to get a new one.


The wood is pine, and I plan to prime and paint it. it's not the most sturdy wood, although it's not rotting or falling apart, I was wondering what I could do to reinforce it and make it stronger. Would adding brackets help? or a product on the wood itself before I prime and paint it? It will be kept outdoors, so having it weatherproof would be helpful too.
Any suggestions on what I could do for the base to maybe make it more solid? add a wooden base inside as it's currently empty? the structure itself will be on concrete.
I also want to add some sort of roof replacement (doesn't have to be wood as originally) and considering PVC (from our patio as we are replacing ours) or a tough textile material (any suggestions welcome!).
Any other suggestions very welcome!
Thanks in advance,