Do NOT use gadget bombs or icbm's - they leave radioactive rubble that can not be removed.

tip for military:

i did a game where i was just total military, i remember that you have to act fast. cities spring up on your continent, and its hard to take them over because theres usually 1 or 2 other civs trying to take them. i ended up allying the cities (there were 2) being taken over, and then when the warlike cities tried to take them, i started destroying their cities, so they would call off the attack, then i would attack their tanks and turrets, then destory their city. my ally died but i knew that would happen. also, always attack turrets first. otherwise you will take MASSIVE casualties. and once you've taken over your continent, boats, boats, boats. sell all your tanks, as nobody can attack you by land now, and create a huge army of boats. by now the spice geysers dont matter, dont take them over, make money from your own cities. just attack the smallest color and eventually there will be another big color about the same size as yours or bigger. you can try to ally them if you have 6+ cities... i just used a gadget bomb on one civ to get 5 cities (it was actually kinda funny... they were attacking my ally who had one city, and i nuked my ally right before they took the city so it counted as mine... but now the big nation hated me...), then i just launched a massive attack on one of the big nations cities to get six (i went from being their friend to being at war with them, but that didnt matter). then i launched the ICBM and got the conclusion achievement. then you wanna max out all your cities before space stage.

maybe you can take some info from this... hopefully it'll help