Teamwork, resources, environment, and planning.
Defenders have a tremendous advantage. They don't have to carry stuff in. They don't have to scout. And they have reserves.
Defenders with an established structure have all sorts of capital:
- Human Capital
- Infrastructure
- Temporal capital
Human Capital
Human capital is the first trick. An adventuring party is fundamentally limited in the number of people they have. These people have relatively limited resources attached to them. There are expenditures, but the nature of adventuring life is that most resources are dropped into incredibly inefficient gear because of the number of hands available to hold it.
By breaking away from the adventuring life and investing in people, you fundamentally get more actions per round. Now, while at much higher levels a capable tier-1 caster laughs at a horde of underlings, there are many things to be feared from a structure that communicates.
At the end of the day, hiring guards based on their ability to Use Magic Device and issuing them all wands of Magic missle is a huge advantage. You don't need to worry about accuracy, and 4 people with a wand of magic missle are far far cheaper than a high level wand.
More to the point, by having purely-defensive people in place and allocating them their own resources for defense means that you don't even need to worry about defensive measures for your offensive folks.
What's even better is that these resources sit around. From the relative cost scales of higher-level gear... if you need to use these resources, you'll probably capture more than their value from the idiots you're using them against.
As a means of insurance, having spells to take care most of the more common problems (adventurers being top of the list) you can amortize the risk and therefore the cost of adventurers over weeks and months, instead of having to pay for the acute cost of repairs.
From a technical point of view, you want to invest in level 1 stuff for most of your mooks. One shot potions, poisons, and wands are absolutely fantastic for this sort of thing. They sit around until they're needed, and there's no worry of "should I save it for later."
Details on loadout:
- Wand of Magic Missle
- More
Infrastructure
Infrastructure is a kind of investment that is nominally impossible or not particularly useful for adventurers. There are different roles for structure, but protection and subdual is the critical aspect here.
The critical thing here is to allow them an "out" so that they aren't forced onto deadly ground. Instead, the architecture should make it harder to go to important places and easier to go to exterior places.
Of course, these exterior places, not being frequented by the public, can have various man-traps (sally ports and whatnot) to thereby contain adventurers in a safe (to the infrastructure) location for handling. But giving them an escape route into this area is critical so that they don't stick around and damage important bits.
Therefore, infrastructure is something that can take as muh money as you want to put into it, and will repay the hotel handsomely.
Features of infrastructure are a function of the role that you want the infrastructure to play
Roles of infrastructure
- Client Protection
- Aggression channeling
- Disturbance containment
Temporal capital
There are two types of temporal capital. Planning and the action economy. The greatest luxury that your environment will have is the ability to have a very short OODA loop. With a plan, the reactions of the opposition will get inside the adventurer's OODA loop leaving them with the feeling that they just need to di di mau because things are simply moving too quickly and the opponents are too well organized.
By having quick reaction forces in place with plans and alert signals, the hotel will completely violate the adventurers OODA loop and social construction of "dungeon." (as the idea of dungeon does not have mutually reinforcing and escalating waves to force people along a desired path. Because fairness.)
Temporal investments
- Signals
- Plays
- Graduated response
Check out this question: How can I play monsters and NPCs up to their potential?
The challange and XP ratings assume (I assume) that monsters play to their strengths. For example, your Hobgoblin captain should be commanding the hobgoblins - these are not mindless drones who attack without a plan - these are genetically professional soldiers. Good soldiers do not fight fair - level playing fields are for sporting fixtures, not battles.
The hobgoblins should refuse combat unless and until they have the advantage or they are desperate; but these guys plan ahead so they don't get desperate.
If the hobgoblins are aware of the party's approach then they should be preparing a series of ambushes, fall-back positions, escape/disengagement plans, booby traps, ways of separating the party and destroying them in detail.
If they are unaware then they should have done this anyway - good soldiers do.
For example, an attack on the party during a long rest has possibilities. 7 arrows with surprise at the PC on watch should render them unconscious with a high probability of success. They can then move in and slaughter the remaining PCs. Played this way this is certainly a 2x Deadly encounter. If they survive (which is unlikely) they will have earned every XP several times over.
This assumes that the party really wants "near death battles that bring them out bloody and bruised". My guess is that this is not what they really want.
The suggestion of a "battle arena" test is a good one. This can also be done in the campaign - maybe there are gladiatorial contests in your campaign that are not "to the death". Playing a team knock-out (that ends with 1 team knocked-out) could teach both you and your players what "deadly" can really mean without having a TPK.
Best Answer
Firstly any environment you can imagine can add to a fight.
As such there is no exhaustive list, however the following resources (it was resources that was requested) should be of use:
But possibly the most comprehensive aid on this topic I've found is this;