Assuming you are butting the back of the cabinet to a wall, you can use silicone caulk as a strong adhesive to secure atop the base, and to secure the c-top to the cabinet. The screws used to secure the cabinet box to the wall will hold the entire assembly from moving. Some installers use screws to attatch cabs to base platforms & c-tops to hold in place til adhesive cures, we don't.

When you say "base" I think you mean a toekick or riser so the cabinet sits off the floor, no?
If you made this from 2x stock, and have shimmed it level off the floor in both directions, and can secure the cabinet itself to at least 2 studs, there's really no reason to worry about fastening them together beyond a few finish nails. If you had a couple of 2 1/2" screws, you could screw through the floor of the cabinet and catch the base. If you were wrapping the base with molding, you could lip it slightly onto the cabinet trapping it.
Casey

Thank you both for your responses. The silicone caulk is a great idea and will use. Yes, I was referring to a toe kick. I do have a 2x4 which I will try. Are there any tips to shimming it level because I tried that once before and was not to successful.