This is probably one of the reasons why I can't win on Noble. Whenever I complete a certain tech which I beeline to reach my goal, I can't immediately access the tech that's adjacent right to it, as I'd need to research a couple other lower techs first. Can anyone explain how the tech tree work?

Each tech, excluding the first group (Wheel, Mining, Mysticism) have two types of prerequisites, optional and required. To research a tech, all the required prerequisites must be researched, along with 1 optional prerequisite if there are optional ones. IIRC, all techs have at least two prerequisites.

That makes sense, but how can you tell which is the second prerequisite since it's not written anywhere?

As empiremaker says, prerequisites come in two types; optional and mandatory.

Optional: These are indicated by the arrows in the tech tree. To research a tech you must have at least one of the techs with arrows leading to it (if there are any. Some techs have no optional prerequisites).

Mandatory: These are indicated by small icons in the top right corner of the tech box. Mouseover them for the name of the tech if you can't recognise them. To research a tech you must have ALL the techs indicated by this type of icon.

Ah, I see now. Thanks a lot, I've always thought the arrows are the mandatory ones. I'm actually surprised I didn't notice the tiny icon on the top right, guess I wasn't looking hard enough.

Also, check out the Tech Tree for Civ4 from the main site. An advanced version has prereqs mapped.

Ah, I see now. Thanks a lot, I've always thought the arrows are the mandatory ones. I'm actually surprised I didn't notice the tiny icon on the top right, guess I wasn't looking hard enough.

No, it's more a case of a lousy graphical presentation.