Vin,
Yes, fastened* to the top course with screws and plugs.This is really to stop lateral movement during assembly , unless challenged by a tornado, I dont think lifting is likely.Always have been in a relatively sheltered location.
Located initially so that the screws may be tightened after glazing, otherwise its easy to pin it down out of true.
Taller cold frames come together if, as I was fortunate to secure ,for nix, a quantity of 12 identical glazed 36"x18" wooden sashes while at the council tip. The donor not only unloaded/loaded them for me,he also apologised that they were slightly faded in paint!!