Military research

There are tiers for the military. For barbarian factions, it is like this

1st tier: Forest
2nd tier: Warband
3rd tier: Light
4th tier: Medium
5th tier: Heavy
6th tier: Veteran
7th tier: Elite

For civilised factions, it is like this

1st tier: Levy
2nd tier: Light
3rd tier: Medium
4th tier: Heavy
5th tier: Veteran
6th tier: Elite

For boats, it is a little different.

For barbarian factions

1st tier: Longboats
2nd tier: Light Warship
3rd tier: Medium Warship
4th tier: Heavy Warship
5th tier: (Insert your faction name here) Warship (Note: to save research, elite warships for barbarian factions will just be named after your faction. For example: "Saxon Warship")

Civilised factions

1st tier: Perikonoi
2nd tier: Bireme
3rd tier: Trireme
4th tier: Corvus (Rome only), Quinquireme (Carthage only), Heavy Trireme
5th tier: Polyremes, Xanthos (Greek factions only)

Transports are divided into three tiers which are as such.

1st tier: Light Transport (Transports 300 men)
2nd tier: Medium Transport (Transports 600 men)
3rd tier: Heavy Transport (Transport 1,000 men)

Now then, onto the topic of research regarding military. This excludes the Roman factions.

Players need to take note of what their highest tier starting unit is. If your highest tier unit is say, a Medium unit, then you cannot recruit any units above Medium from the start. So, you must research Heavy Units before you can begin to make them. Any unit in your default starting army can be upgraded. So, if you start with Forest Spearmen, you can eventually get Elite Spearmen. The same applies to boats.

I also forgot to mention, that if your unit doesn't have an assigned tier next to it, this troops fits into Medium tier. These troops cannot be upgraded, and are recruitable always as medium tier units.