Ancient and Medieval battles: going deep vs going long. Isn't going deep a terrible idea?

Lines were never 100 or 500 men deep except in the most cramped of circumstances, and never by choice. On the very thick side you would go up to maybe 40 ranks deep, and on the very thinnest as low as 4. Late Roman and Byzantine armies tended to have two main lines of heavy infantry, each 8 men deep, which could be combined into one line 16 men deep when needed. The practical difference between a line 100 and 500 men deep indeed wouldn't be that big, aside from the massive waste of soldiers, but the differences between lines 4, 8, and 16 men deep could be monumental. I think anyone would be more likely to give up ground or even rout if the enemy in front of him had 15 guys backing him up and they only had 3, particularly if those 15 were lobbing projectiles and in the closer ranks getting the occasional stab in.

Deepening the line definitely made a real difference when you're talking about files of 4 vs 8 vs 16 men -- not so much when you get to 30+, as the commanders of the time realized ("If for some reason you want to make the files thirty two deep -- not a very useful idea --give the command: "File after file:", from Leo VI'sTaktika), but before that point it was often decisive. You could stand to lose a lot more men while holding the line with a depth of 16 men than 8, and likewise for 8 and 4, while trying to hold back the assault of a line with files of 16 men with 4 would be nearly impossible. The "pushing" itself was largely a byproduct of the inherent inferiority of thinner lines, up to a point, and the fear of the soldiers therein of being hurt or killed because of that inferiority, accentuated by the deaths or incapacitations of those around them as combat progressed -- it wasn't a physical push in most cases, but it was the logical response of a being interested in self-preservation to the sustained attack of an enemy with many more men and many more resources.

Well yes, and that's exactly what militaries disciplined enough to maintain such a formation did -- a second line was detached from the first, and kept far enough away that it wasn't as vulnerable to arrow fire but close enough that it could reinforce the front line in a snap if needed. Having the infantry push the enemy back was often less important than having that reserve line ready, particularly in armies with a strong cavalry wing. The double-line formation was a strong compromise between holding power, adaptability, and security used by the Romans for well over 1000 years.

Interesting, I forgot to disclose that I need this information for early development of a realistic yet elegant (if I do say so) tabletop war game. There are several conditions that trigger a unit to rout, one being if its health becomes 1 and it is engaged with one or more units whose total health is 3 or higher. I think now I will edit it to counting the health points of units of the same army behind one another. I was trying to give strategic motivation to go deep, yet I couldn't think of any and as such there would be no reason for a player to go deep. With the game mechanics, there are several other reasons you might want to go deep but they are negligible and don't outweigh the risk of envelopment.

There is a simple pushing mechanic which would mainly be used to perform a squeeze and originally I thought going deep would prevent pushing, but then I had the same thoughts as I've explained and how it wouldn't really prevent pushing and so one unit can push an unlimited amount of units who are behind the unit it is pushing. Is this realistic, and how might you change this? Should going deep make pushing more difficult for the enemy? Or should it just act as a way to reinforce the front unit and prevent routing?

Also, current prototypes for the unit pieces are 2 x 1 inches, according to you and several other people on this post units were about only 8 men deep but 100 men long, so should the pieces reflect this and be more like 4 x 1 inches or something? Or would you just leave that alone because it's not meant to be fully realistic and the unit pieces only abstractly represent what a unit would be, especially considering some units were square in formation? I appreciate the help, thanks.

EDIT: I should state that pushing is optional and can be done when rolling a hit on an enemy unit, so should it be harder to land a hit on a unit that is back up by other units? That just doesn't seem very realistic to me.
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Jan 2015
So why did ancient and medieval armies not know this?
Never ever make an assumption like this. If an ancient army did something that we don't understand, that's because WE do not understand. They were all too familiar with exactly what was going on, and with what worked and what did not. Screwing around with proven formations and tactics could be mass suicide, so they didn't do it. They *could* branch out if their troops were trained and disciplined enough, or under other very rare and specific situations.

But considering that we still can not prove *exactly* what happened on a man-to-man level when 2 armies slammed together, we simply can't make too many solid conclusions about many aspects of battle.

Nov 2010
Just to throw in another grenade - the OP refers to medieval. Those of a fundamentalist disposition - eg Almoravids and Almohads - would use a sort of cannon fodder in front of their own lines - in the first case unrelaiable allies and in the second case 'jihadists' - poor expendable, mostly unarmed, unarmoured and mostly unfed (by the main supplies) volunteers daft enough to die for the faith. Crudely speaking putting a mass of these in front of the lines gets the heavy knights tired out hacking at them and getting tangled up in them. So when they reach the actual line there'a big disparity in energy levels.

In the middle of this the African light cavalry could outflank the tired Castillians, surround them and ultimately attack the camp. Time after time after time!