Here is a build that I call a "Screenhouse." There is no heat control, so it is not a greenhouse and it can be called a cold house, for I can put a tarp over it if I need to. But this screenhouse is covered in 1/2" hardware cloth to keep all the critters out. We have the worst squirrel and rat problem in this area, because of a wild field next door. The squirrels have eaten flowers, cucumbers, rose bushes, luffa plants, peppers, and of course tomatoes!

This is 8 1/2' x 14' with an arched roof, that I would construct differently. It was really hard to get the roof on with the adjustable hinges. The doors were attached with hinges made for pvc 3/4" pipe, including the closure (open to better solutions). All vertical conduit pipe are 3/4" and all the horizontal conduit pipe are 1/2" in diameter.

There is a sink with a conduit pipe stand and shelves attached to the vertical pipe. There is also a hanging rectangle that string trellising are attached to. You can see it in the last photo.

So far it has kept out all the critters!