Como alguém lida com um gargalo de inimigos na porta?

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.

You are correct in your rules understanding. There are of course missile and reach weapons, though in this situation both usually will incur a cover penalty to hit. And splash weapons, and spells - but this definitely isn't a rules exercise, it's an exercise in tactics.

A bottleneck isn't necessarily bad. Keeping enemies bottled up at a chokepoint allows fewer of them to get into the fight and lets you pound on their lead guy more. It favors the defenders and those that have options other than pure melee to bring to bear.

Tactical options here for either group basically include staying there and slugging it out or backing off to open up the battlefield. And then the other group has the options of moving into the breach or not. Sometimes it ends up with both groups backing off and shooting at each other (or attempting to re-close and block the door). In many situations (the usual dungeon or keep, more-bad-guys-than-PCs setup), it's in the hobgoblins' best interest to try to slow down the action, make a lot of noise, and attract allies.

A smart tactical move (and hobgoblins, of all the humanoids, are the ones known for their tactics) is to move back a little to tempt someone to come in through the door - and be exposed to up to 5 attacks. I had a PC party that overcame the usual "charge in" mentality and decided to play more tactically, and they had a lot of success in luring low-self-control bad guys out from a room to where they're surrounded.

If one group thinks they're totally more bad ass then they might just open up way more, form a gauntlet or just spread out and try to lure the other group in for a more timely whupping. Also, spells and splash weapons tend to affect closely bunched groups, so they may try to take that into account in their positioning depending on the situation.

Both you and the players will learn as you go about more effective combat tactics. But it's OK to just say that if the party really wants to not bottleneck then they have to back off or use something that'll change the situation (flaming sphere!) and same thing for the monsters. This might also teach your PCs not to stand right outside a door and have a conversation about going in and killing whoever's in there, unless it's a really thick and soundproof door.

There's a few options:

1. Don't put monsters near the door. Rather than having the hobgoblins open the door, have them hide behind cover ready to ambush the players if they come in. Put them around a corner, or make the hallway wider such that bottlenecks don't happen.

2. Have the monsters use tactics to break the deadlock. Grapple the character in front and pull him out, or try to push him back into the previous room. Some monsters might have natural options for this, for others just use the rules in the PHB for grappling and pushing.

3. Have encounters with fewer monsters. Not much point in fighting, say, a single Ogre in a doorway, and staying lined up like that against a Dragon is suicide. Varied encounters will naturally prevent this kind of thing from coming up too much.

Overall, though, I wouldn't worry about it too much. As you noted, the players still had fun, so there's no problem with having the encounter work out like this every once in a while, as long as it doesn't become a sure-fire tactic they can use every time.

It can be tempting as a DM to try and counter everything the players do, but a lot of the time, it's best to just let them get away with their crazy tactics that break your encounters. It may seem like the effort you put into the encounter is wasted or that they didn't get to experience what you wanted, but often "breaking" an encounter like this will be much more fun and memorable for the players and feel like much more of an achievement than if they'd simply fought it out normally. Of course, if the same tactic works again and again it becomes dull, but every crazy strategy should have at least a chance of working once.

This is an answer to the question:

What strategy can a DM use in a combat scenario like the one above to make sure that the NPCs are all contributing to the battle. Especially if they have no ranged weapon, is there a way that they can all try to attack?

A strategy I am surprised no one else suggested - have the NPCs take turns holding the doorway. There's no reason to have one hobgoblin standing there soaking hits for half an hour while his allies sit around, then he dies and another one moves up. Instead, have each hobgoblin take a turn being the one taking the hits.

And with D&D 5e's free movement between attacks and through allies' squares, it gets better. Note: this only works if you use the standard of having groups of NPCs move on the same initiative. Have a hobgoblin move into the doorway, attack, and move out again. Then, the next hobgoblin does the same thing. With clever positioning, you should be able to cycle the whole group through.

If you have 50 hobgoblins, they can can each attack in a single turn.

Note that attacks of opportunity are meant to interfere with this, but the PCs will only get one per round. Not really a big deal when compared with having all of your NPCs fighting. Also, you should obviously leave the last hobgoblin standing in the doorway.

If the PCs have one character holding the doorway, he won't last long against this sort of strategy. Instead, your PCs will start taking measures to prevent this situation or completely neuter this kind of strategy.

For example, a single Spike Growth spell would cause this strategy not only to fail but actually to cause the death of any group of monsters who tried it. Similar results could be achieved with a simple flask of oil. In fact, a flask of oil is even better because it doesn't just prevent swapping creatures through as outlined in this answer, it prevents a single creature from sitting in the bottleneck. So you won't just be improving your tactics, you'll be improving theirs.

In any situation where people are clustering together Area of Effect is your friend.

Spellcasters are the obvious choice but even mundane enemies have a number of options, for example if the hobgoblins start throwing burning oil then the players are going to be reluctant to stand still.

One Hobgoblin with a tower shield in full defense backed up by others throwing splash weapons over his head is going to rapidly become painful.

Alternatively use the choke point to your advantage, have the hobgoblins step back and form up around the hole. Their archers get clear shots out - while anyone coming through the doorway gets attacked from both sides by two or more waiting hobgoblins.

If you have the resources things like backing that up with a grease spell can put the players in a really bad situation as their lead member gets pounded on while anyone trying to come to the rescue slides around on the floor.

Let the players then come up with a solution to this problem. The important thing though is that you go with whatever idea they come up with. You gave them a sticky situation, reward ingenuity in finding a way around it.

For example in similar situations in my game a (very high strength) fighter has bashed a hole in the narrow bit of the wall where the murder holes were. In another situation the low dexterity dwarf was lying down and being pushed across the grease spell, then helped to stand on the other side.

You can't willingly end your move in an ally's space, but you can break up moves between attacks and even avoid attacks of opportunity with the Mobile feat. Depending on initiative order, mobile strikes could be used to attack and then disengage so that other characters can jump in or ranged characters can have a clear line. Mobile attacks might also be effective at kiting enemies away from the bottleneck point depending on the layout.

One more tactic for allowing more enemy NPCs to participate in a battle:

While the PCs are engaged battling heavily armored hobgoblins at the door, ask them to make Perception checks to determine if they notice the secret door behind them slowly opening . . .

. . . revealing a second contingent of hobgoblins who pour into the room. Or alternatively a sturdy gate behind the secret door allows the 2nd attack force to safely shoot missiles at the exposed PCs.

This seems like a slightly dirty trick, but it makes sense that crafty hobgoblins would have constructed secret passages to bypass chokepoints and outflank invaders. This also encourages players to inspect rooms for secret doors.