Building permits! I know, it's not a fun topic compared to installing the tower, but you'll save yourself a lot of grief if (a) a local code enforcement officer happens to notice (this is a big, big revenue generator in California); or (b) the tower collapses on a nearby structure, gets hit by lightning and burns your house down, gets hit by lightning and burns the neighbor's house down, etc etc etc.
I went to my city hall for permits for my 38' tower… they were very helpful. For the concrete base I used a welded rebar cage purchased from a local building supplier, they had the cages in stock for about $80. These are the same exact cages used as footings for structural concrete pillars in commercial buildings.
To level the tower I used a Starrett "Mechanic's Level" purchased from McMaster-Carr for around $45. It is much more precise than a carpenter's level, and detects a tilt of .005" per foot. I wanted that tower to be DAMN plumb, and it is!!
Also make sure you orient the tower hinges toward a long area of free-run so you can lay it down… if you use a hinged tower.