I recently found this on the internet and thought something like this was needed on the alliance page so that people who want to become turtles have an idea of what they need defenses wise.
I have never played as a turtle myself but i build considerable def just so i can be a bit more relaxed when not on my computer.
Here is what I found:
Here are the defenses that every turtle should have on each planet (These numbers are the absolute minimum!)
RL: 2000-2500
LL: 2000 - 2500
HL: 500-600
GC: 250-300
IC: 500-600
PC: 50-100
SSD: 1
LSD: 1
ABM: max. your Silo can hold
~ Docta
EDIT: Of course you need high mines, shipyard level 8(+), a rather high robot factory and maybe a nanite FIRST. Else it'll take you ages to do what you could do in a couple of days.
Hope this helps