As for the legion structure, it's fairly complicated and of course it did change over time. However, this is it :

General, or Legate in overall command of approx 4,200 men.
The numbers could vary considerably, especially as losses could reduce a legion strength over years whilst still keeping the main structure.

Senior Tribune came next - one of six appointed to each legion.

Then the Praefectus Castrorum - in charge of the camp, third in command.

Then the other five equestrian tribunes.

Then the Primus Pilus Prior - the most senior centurion, who commanded a century of trirarii legionaries. (More on them later.)

Then the other centurions - I can find no mention of cohort commanders, so the next stage is the centuries and maniples. Each cohort of 420 was composed of various different kinds of specialist fighters. Legionaries did not all look the same! The Velites were often young, fast and lightly armoured, the Hastati were more experienced, with small plate armour and a greave on the left leg. The next group is men in the prime of life, the Principes. Lastly, the trirarii were the crash troops - veterans, very experienced and well armoured.

Each maniple would be made of two centuries, though they worked together as the smallest tactical unit. The twin centuries acting as a maniple would have a Centurio Prior and a Centurio Posterior - the first higher in rank than the second.

The legion of 4,200 was divided into 30 maniples of these four types. That makes sixty centurions - thirty of whom would sit in council with the general and the tribunes.


There would be various non-combat men attached to maniples, known as Principales. Surveyors, etc. They bring the overall number up considerably.

Each centurion would choose an Optio, who used staffs to wallop the troops if they were out of line. Sergeants seems a reasonable modern equivalent. They would also nominate a standard bearer, which was a position of some respect and usually given to the best, bravest men. Each maniple would have had a trumpeter and hornblower, also.

Centuries were never a hundred men - the confusion comes about because two of them made up a maniple that could have 120.

Below that lot - the spine of the legion - we have a watch officer for each century, who would have looked after the password of the day on a little piece of wood. A low rank. He was selected from the tenth maniple of each class of infantry and excused guard duty.

Also, 300 cavalry were attached to each legion. These were split into 10 groups of thirty called 'ilai'. Each Ilai would have three officers for ten men and these would choose optios from the other nine. The optios would ride at the back of a file of ten, the Decurion at the front. Small mobile units is the key to the whole Roman war idea.

I think that's about it, though I couldn't find the book I wanted to check the order of those tribunes. It turns a solid block of five thousand into a fluid wave of smaller groups. Even the centuries were split into tent-mates who had to put their tent up and sleep in it together. That made for very strong bonds - you just can't run when your friends are watching you, men you've known for ten years or more.

It is worth remembering that various changes occurred to this structure over time - and innovators like Caesar used them in varying ways. The extraordinarii usually led legions on the march and had the task, on foot or mounted, of holding off a hostile attack while the fighting squares formed. They were the crack troops used for special assignments. They also guarded the area when camps were being made.

That's it, Sean. I'm a little worried I'm wrong about where the Praefectus Castrorum comes in those top ranks. I think he was third in command and I seem to remember his was the highest rank a soldier could dream of reaching without political power and wealth. That said, it was a very prestigious post and very well rewarded.

I really should get back to writing book four now.

Conn Iggulden