That’s the big destructible prop that’s used for the explosion animation, right? It looks like the prop isn’t actually enabled until the third stage starts, which’ll never happen in GMod since the gamemode logic doesn’t work.
You can probably force the model to show up anyway by doing something to it with the ent_fire console command. I’ll go take a look at it in-game and see if I can find a solution you can use.
[editline]10th August 2015[/editline]
Okay, the best thing I could find was ent_fire explode_anim turnon;ent_fire explode_anim_tracks turnon, which enables the models for the “destroyed” version of the tower. Since it doesn’t play the explosion animation, it pretty much looks the same except for a bunch of cracks in some of the planks (particularly the part right at the top).
That’ll have to do, since the entity used to spawn the non-destroyed tower doesn’t seem to work.