I wouldn't focus too much on defensiveness unless I border a warlike or a backstabby civ, especially if you play below immortal difficulty level. You must remember that focus on something will decrease your effectiveness in other fields so if you move your city a little to get better defense, you'll more likely deprive it of some good food or production tiles. Also, if you really need to worry about being jumped by your neighbor, it's not someone who you could fend off with 2 defensive units.
2 vs unlimited number is not a good battle forecast.
As with everything in Civ, it comes to the difficulty setting. If you are playing on king or below, just focus on food and access to at least one new luxury resource. On emperor, make also sure that there will be some production available. On immortal, your city should be in a position you can defend, i.e. you are able to limit the number of units that can approach your city in the same time. AI uses brute force rather than even the simplest tactics, so you need to be able to hurt them from afar and fortify yourself in some sweet spots. Deity is similar to immortal, though you'll have much less time to settle your desired plots and if you start near Poland, Hiawatha, Kacapy or Shaka, you can just restart your game
>Edit<
Oh, maybe three more general things.
1. River is always better than no river. Also, mountain-neighboring tiles are very useful for the 50% science bonus from observatories but remember that observatories come pretty late and you also need to build them. On one hand, if mountain would be the only benefit from a certain location, it's not worth it, especially not in early game. On the other, if you combine NC and observatory in one city, you'll get huge boost for your specialists and academies.
2. Coasts in BNW are extremely important for trade routes. If these are also rivers next o the plot you settle, you'll be getting 25% bonus to trade routes value. Regardless, two early cities connected with naval food routes will grow faster than sand people population in Europe. Especially useful for cities with guilds.
3. I forgot what I wanted to talk about here. Maybe it'll come to me later.