First, I'd like to say Hi to all. Also, thanks to the devs for this great game! It's like the best of Civ and MOO.
Something I have found that's related to some old posts - Duranthium is better than Tri-strontium which comes after it.
Super Duranthium: size 5, cost 70, defense 4
Tri-strontium III: size 5, cost 70, defense 3