[Unsolved] Cannon Tower

I'm new to this whole thing so excuse me in advance for anything noobish...

I'm having a few problems with my first map, I'm trying to make a sort of modern war thing and the heavy artillary I'm trying to make is getting to me.

For one, I'm trying to use the Cannon Tower as its model, for some reason its not letting me. I even tried modding a steam tank to appear as a connon tower but that still didnt work and also makes it mobile...

Second, I want to make it to wear you can only build these near your own base. I dont want it to be possible to bomb the enemy base without relent...

And lastly, for all my ranged attackers, can anyone give a good hint to calculate distance? 500 sounds long but it really doesnt seem that way. :/

Thank you!