This part of [Introducing Late Night Fighting] certainly owes a huge debt to games like Wizardry and The One Ring, and their latter day pastiches like The Dark Spire.

Introducing Late Night Fighting uses "mêlée groups" to determine who can hit each other. You maybe don't know where everyone is down to the inch, but you know who is tussling with who.

So if you want to protect someone from swords and axes, you place them in the back rank. This is how it works.

There are three ranks. You can get shot just as easily in all three ranks, so the ranks only affect your own shooting, and mêlée attacks.

Front rank, you can make mêlée attacks normally. Shooting is trickier: Front rank PCs shoot at a disadvantage, and front rank monster NPCs need to select mêlée options if they have them (if they only have ranged options, they are at disad).

Back rank is more cozy. You can shoot freely and you can't be hit by normal mêlée attacks (nor can you make them). But it's tricky to qualify for this cozy position. You can get there by being hidden, being in an unreachable place (bookshelf climbing), using <b>Levitate</b> or similar magic, or by being <i>guarded</i>.

Guarded isn't a rank, it's a way to qualify for the back rank. You can be guarded if there are fewer than two monsters per hero total and there's either at least two front rank heroes guarding you, or at least three front rank heroes for each monster. (This kinda clunky formula is probably the most direct influence of The One Ring on these rules. We've just stuck with it.)

The guards decide who they protect if there’s an argument. Conjured creatures can’t be guards. You also can’t guard against swarms or incorporeal movement.

Enemy NPCs use a different way to qualify for back rank instead of guarded. They can hide or climb or levitate as normal, but they don’t need to be guarded to be back rank, but they can only be back rank if there is at least one front rank monster for every front rank hero.

You can switch ranks even without having or using any movement speed, unless you're restrained or grappled, as long you otherwise qualify (for example, you can become guarded even if you're unconscious on the floor).

The third rank is called free air. You need full flying speed, not just dinky levitation, and just like the back rank, you can shoot freely. You can only be in "air" mêlée groups (i.e. other flying creatures) so if you wanna make mêlée attacks, you need to fly low enough to just simply be in the normal front rank as opposed to the free air rank.

Reach attacks are awesome! A "reach attack" isn't defined in 5e but here in Introducing Late Night Fighting it means if you have longer mêlée reach than your target's longest mêlée reach attack option. So bring out those lunging attack maneuvers and those whips.

To make reach attacks, you can't be in a mêlée group, you need to stand alone (in other words, closing in on someone is a good way to prevent them from making reach attacks), but you can be in the front or back rank and so can your target.

If you have a speed of 25′ or more (a.k.a. 75 in many OSR games like B/X), you can move in and out of any amount of groups on your turn as long as you're in the same area / room generally.

If you're slower than that, you can either join a group if you're unengaged (even if you just became unengaged by defeating your enemy) or you can leave a group if you're not. (I got this rule from how the boat works in the old "Container" board game.)

You can use your reaction to make an opportunity attack on enemies in your mêlée group if they leave your group without properly disengaging.

If you're making reach attacks on someone that's not in your group, you can use your reaction to make an opportunity attack on someone if they do any group leaving or joining or switching except if they close in and join a group with you.

So how does this system interact with protective abilities? For the most part, it complements them well rather than change the rules for them. I'm talking about the fighting styles Interception and Protection, or the defender sidekick's martial role ability. All of these can help another front rank ally who is in the same group along you, which can otherwise be very dangerous since monsters in that group can focus all of their attacks on one hero. We've found them to be pretty strong.

This isn't part of our current Introducing Late Night Fighting rules, but just for design history purposes.

Before we started using this rank system, which owes a direct and deliberate debt to many classic games, I had independently come up with something I later found was coincidentally very similar to "Intercepting" in 13th Age.

Here's how they put it:

You intercept a creature when you move to stop an enemy attempting to rush past you to attack someone else. You must be near the enemy and the person that enemy is trying to reach.

We found that this wasn't enough in mitigating the danger of enemies focusing their attacks, so instead I gradually developed Introducing Late Night Fighting.

Reading the 13th Age SRD now, I sure see a lot of other similarities! "Engaged" vs "free" for example. In my mind, all of this is lurking emergently just under the surface of 5e's rules and I just unearthed those play patterns, making them explicit. 5e is very clever, but I think they probably read or were familiar with 13th Age.