[RPG] correct way to deal with “fish-in-a-barrel” type combat

You're seeing one of the classic military ideas play out in your game — control of chokepoints. As many other people have pointed out, the advantage to controlling a chokepoint is that you can step back and force the enemy to push a narrow front of combatants against a broad base of defenders — so the person coming through the door is taking the most attacks.

Obviously, though players (somewhat) tend to be better disciplined than most people were in real life — players are not going to rush their characters through pushed on by adrenaline and machoism to face multiple attackers, but probably back off and attempt to attack at range. This is assuming NPCs with little planning or expectation to be fighting indoors — but if they are? The game changes drastically.

Buying Time

NPCs fighting defensively are often buying time until reinforcements can be summoned. This means they can focus on defense, since their goal is to stay alive and keep the enemy occupied until help arrives. This may also involve using some kind of push or shove action to push the enemy back and slam the door shut. Or, maybe tripping and knocking down the enemy so their allies have to step over them to get to you.

Alternatively, they may choose to retreat from the door, throw down caltrops, or knock over an urn of burning coals, etc. and retreat to the next defensible doorway or position. Knocking over furniture to block the path might also be a worthwhile choice.

If they've already notified the reinforcements, there may be a hidden ambush in the next room, or a lot of crossbow wielders all basically waiting to see what non-ally tries to get through the door.

Or, if there's a way for the reinforcements to loop around the party's rear, they may do that, instead.

Pulling out the Wounded

You have a line of combatants. Behind them, are their replacements. When someone is wounded, an ally pulls them back and shoves them to the rear, while another person steps up and fills the gap. This requires well trained troops used to formation fighting, although, by usual D&D mythos, hobgoblins are pretty organized fighting units, so this would make sense.

Although in D&D there's no real wounding or ability loss until one hits 0 hitpoints, it might be worth remembering any surviving hobgoblins can report back intel on the group, and they can plan better — who are the spell casters? What kind of fighting styles does the group use? This is even more important when you realize that the enemies will start bringing countermeasures in tactics and defensive tools to help deal with the enemy.

Indoor Weapons

Spears are not universally good indoors, but they can make controlling a hallway or doorway a very viable option. In real life, you can get many spears lined up, two or even more lines of troops aimed in a small space. If you're using the grid rules in 5E that is more limited than real life options, but still doable.

Darts and crossbows are pretty great indoor weapons for ranged options. Short bows might also work too.

Defensive Architecture

A lot of castles throughout history had arrow slits — small openings in the walls so archers could shoot through at intruders. These mostly lined the entranceways near gates, though dungeons might have them in other places. Of course there's gates that fall shut, barred doorways and a lot of options along those lines.

There might be areas with traps that can be activated by levers or pressure plates, and the bad guys will try to steer the characters into them, or at least, use the threat of these things to slow them down and keep them at bay.

Magic and unusual options

D&D being D&D, there's a lot more options beyond what you get in real life. There's obvious spells to create webs, walls, or lock doorways, but there's a lot of other ways magic can become very tricky.

• Summoning a swarm of bugs or rats to crawl (over, around) the party in the doorway, and attack or at least disrupt spellcasters.
• Using darkness, or an illusion to make it look like the enemy still threatens — while they've actually retreated from the doorway and are making their way to somewhere safer.
• Grease spells on stairs, or narrow walkways.
• Illusions of floor hiding pitfalls, making closed doors look like blank wall space, etc.

Three steps to deal with this, although step zero is probably “yes, don't worry about it so much.”

1. Make sure the action is actually possible first. A character only gets one free simple interaction with an object, such as closing or opening a door, or drawing or sheathing a weapon, or jumping up on a table. Pulling out a rope (interaction #1), tying it around a post (#2), grabbing someone (action), and jumping out the window (interaction #3 maybe? + movement) is more than can be accomplished in one turn. And carrying a fellow adult — even a cooperative one — is non-trivial, so make sure that's not happening for free either: pull in the encumbrance rules and penalties on movement speed to see if they can actually jump out that window this turn, or if they maybe end their turn perched precariously on the windowsill, just in time for the thugs to get their turn.

   Also note that if the hero is physically blocking the thugs over two turns in order to fit all the interactions in, the thugs will naturally be adjacent to him and will get opportunity attacks on the hero (but not the damsel) when he moves out of their reach by jumping out the window. He also sometimes won't effectively block them if he's busy mucking about with a rope unless it's a very narrow space, because you need two hands for tying rope; thugs can just move past an unarmed hero without provoking opportunity attacks.

   If he grabs her arm and “leads her away”, then that's not going to work at all — she can't follow his lead without using her own movement, so all he's doing is Readying an action to walk with her when it's finally her turn. (He would have to grab her arm and drag her away, not using her own movement, to make it happen on his turn. And I'm sure the damsel will have words with him after — or during — such a maneuver…)

   But however it works out, let it work out how it should by judicious leverage and application of the rules rather than trying to decide that it shouldn't happen at all.

2. Notice that forced movement is a thing in the game and doesn't break any rules. Initiative is just who gets to act first; it doesn't prevent others later in the initiative count from benefiting from those actions.

   Is this an interesting way to make one character end up farther away than they could on their own? Yes, but it's at the expense of another character spending their turn in a rather sub-optimal way. (Although, sometimes the real goal is better served by doing something “sub-optimal”, so the definition of that is arguable.) Pushing and pulling others around is something you can do with your action, so there's no reason to be alarmed about it being used to improve an ally's position.

3. This doesn't end the conflict. Just because he's moved an ally out of imminent danger doesn't mean the danger packs up and goes home. He jumps out the window with her? Well… quite apart from the risk of dropping someone because you're jumping out a window and trying to hold up the weight of two people with bare hands and no climbing harness (which you might want to talk about, and maybe call for checks regarding), you still have a conflict — just not a combat.

   There's nothing stopping the thugs from peering out the window, shouting “Follow them!” and clambering down the rope too; or dropping rocks, chairs, cannonballs, and priceless bric-a-brac out the window on our intrepid heroes' heads; or hustling down to ground level to kick off a breathless chase conflict.

So yes, don't worry about this. Roll with it, take any necessary few moments to consider how to apply the rules to adjudicate the attempt, find out if it works, and then make the enemy react realistically and interestingly to the unexpected tactic.