Okay, I've already gained access to the armory, & now I'm at the part where I need to set up artillery. I've already got a few gun turrets so how do I build them? I tried building a missle turret, but I cannot buiild them as I need level 3 Science.

Its in special when you go in workshop. Very first tab.

go back to the last room you were led too and pickup the artillery schematic off the table, then its under specials