It's late and I need to sleep, so I'm going to try to be brief here.
First, there are no ships in Thermopylae, so we don't need to worry about blockading it.
Second, Lee, you and I obviously have very different styles.
Let me explain the logic I've been using. At this point, we have five immediate neighbors: Mao, Alex, Gandhi, and Victoria, and Hatty. If you will look at the power graph, we're above all of them but Hatty and Mao. Vicky, Hatty, and Gandhi are all very pacifistic personalities, in my experience: I can't remember any occasions when I've faced a backstab from them. Hatty shares a land border with Huayna, who she dislikes more than us. None of those powers, except Mao, has "too much on my hands." The only more distant power I fear is Isabella, but she's below us on the power graph, doesn't have enough on her hands, made peace with us not many years ago, and has closer borders with other AIs she dislikes. Even if she were to attack, she would hit Yekaterinburg and the Roman island first, not our core, and we could afford to buy time with space, recapturing cities later. Thus, my assessment of the diplomatic situation is that we should expect a war with Mao soon, once he's built enough troops, but not immediately, because he just switched to Vassalage and Theocracy; and that no one else is going to attack us.
I run a very light defense because units sitting in cities don't do any good. I rely on civics, upgrading, and knowing when the AI's going to attack for defense. Right now, in an emergency situation, I think we could whip, and if necessary, switch to Nationhood and draft, enough units to beat off an offensive. We also have a large cash buffer to upgrade obsolete units into real defenders in an emergency.
I think that a war with Greece is going to be very short. Once we take Sparta and Athens, it's over, and we have adequate forces on hand to do that. My emphasis at this point is speed: we need to be gaining ground. Also, because we're racing against the other teams, I think we need to take some risks to do this. My calculation is that Mao isn't ready to attack now, so we have time to build forces before then, and adding Alex's cities would help us. I'd hoped this was a risk worth taking.
Because this is an archipelago map, we have few cities with any real production. Several of our cities are so slow at building anything that it doesn't really matter what they're building. (Pisae, Arretium, and Cumae, for instance.) If you count strict numbers of cities, the numbers look thin, but if you count hammers, with our two best production cities on military, about half our per-turn hammers are going to build military units. As soon as Antium finishes its lighthouse, I think it should start or whip a barracks, and then go onto military from there; at that point, about 60% of our hammers would be going to military.
I can see an argument for building more units in our mobile reserve, but I strongly feel that we shouldn't be garrisoning our cities with anything more than obsolete units, one per city. We know where the attack is coming from, let's fight Mao on our terms and make sure that it doesn't matter what we're using to police our cities with.
