The tech tree in CIV is quite big - 86 technologies do not make it the biggest one in the series, but what is worth mentioning all technological advancements are useful and give significant advantages.
The most important feature of Civilization IV tech tree is flexibility:
- In Civilization III there was a division of tech tree on four eras - the player had to made most of research in one era before can go to the next stage of technological advancement. In CIV tech tree has no such limitation - it's possible to research every technology with prerequisites you already have. That's an opportunity to research perks much more advanced than the other ones you already have;
- The second important feature is more than one way of developing of particular technology - there're different research paths and optional perquisites what gives great strategic implications.
On the other hand there're three factors limiting research flexibility:
- There're some key technologies, critical for further development which you have to sooner or later research (for example gunpowder or scientific method);
- Some units and buildings require more than one technology - for example cavalry needs horseback riding, gunpowder and military tradition. This connection between tech branches makes it impossible to build an unit or a building without the full knowledge of some important technologies (like building airplane without the knowledge how to propel it - so you need not only flight but also combustion)
- The more advanced the technology is, the more expensive it gets. Even if it's possible to start studies on some far more advanced technology than your technological level is that can be extremely long. That's making this kind of technology usually too expensive to research.
Because what and when you need to study is different in almost every game, it is impossible to give precise advice about research strategy. Here I put some more general suggestions helpful in deciding what path on technology tree to choose:
- Some of technologies give extra gains to those who firstly have particular advance. It can take form of becoming a religious founder (more in chapter Religion), or free Great Person, or free advancement. It's a good idea to try take some of these technologies first;
- There're some advancements which are strategically important during wartime. If we received it before our neighbours - we have a significant advantage which makes a small war a really god idea;
- Except of very specific situations avoid to have more than 20 (ancient and classical age)/15 (medieval age)/10 (other) long research times. If it's going to take longer - research something else, and invest in scientific and commercial infrastructure. I emphasise that these numbers are the maximum amount of time, as the average time should be less than half of this numbers;
- The most useful technologies are those which improve commerce and research - creating a snowball effect and making further research even simpler;
- Use effects of scientists' works as quickly as possible. There's no use from even the most advanced technology if it's only in Library instead on battlefield;
- Use of alternative ways researching of expensive technologies - free technologies from tech tree, advancements from GP and Oracle. If properly used - it can make wonders;
- As in all aspects of the game, also in research strategy it pays to focus. Decide what wonders, civics or units are the most important in implementation of long-term strategy and try to find out how to reach these points the quickest way.