I've been building fire towers for thirty years and getting pretty good at it and I've also climbed a few as well. I started out using plastic angle stock used in architectural models and while that worked all right indoors, it suffered some degradation when subjected to the elements. 

When I discovered the screen framing material that I used for the train bridges it seemed natural to try using it for the towers and its worked out well. Following the same construction technique using gusset plates and pop rivets results in a robust structure that stands up well outdoors. The observation platform utilizes 1/4 in. square brass rods and 3/32 dia. rods all soldered together to make the railings. 

The cabin is the only element that uses plastic in its construction since it just sits there and I've been able to install lighting inside to good effect even though it's a bit brighter than would be prototypical. 

And finally, everything gets cemented in place on a mountain of my choosing!