Bad, bad hobgoblins...
A set-the-scene thought...
As the players enter the area, describe how they feel a very gentle breeze blowing towards them...ever so slightly...like a natural draft (more on this later).
(1) Primary mission of the first guard post is to raise the alarm for the rest of the outpost. They will have 1-2 lightly armored "scouts" that run at the first sign of danger to raise the alarm. Being reasonably smart and well-trained, they are likely to have a primary and back-up alarm system, such as a gong or bell in the first guard chamber and another one 30 feet further back in case the first one is taken out by a silence spell. If they are sure the alarm has been raised, the rest of the lead guard post is to retreat (if possible), surrender (if the remaining guards are clearly outmatched, they have been instructed to surrender and parley...offering a false map of the complex - which takes the PCs to a dead-end room or other good ambush place - in a ploy to buy time for the rest of the defense to get organzied) or play dead (this works very well if you have never tried it before and don't have a "I cut all their heads off" PC - have one or two of the enemy 'take a dive' - only to get back up and stick a spear through the wizard during the middle of the next combat).
(2) I agree with the above poster...no straight passages, especially near guard posts and choke points. The design of most medieval tower stairs was a narrow, right-turning twist while going up. This was to allow a right-handed defender to back up the steps while fighting effectively while a right-handed attacker would be at a significant disadvantage (-2 to -4 on attack, no 2-handed weapons). Make the first turn out of the initial guard post a narrow, 90 degree right-hand turn for a similar effect...shields guards from line-of-sight spell effects, allows only one assailant to attack at a time and allows for reach weapons like long spears to be used to "aid other" and gang up on the barbarian. A series of such turns could allow a small force of hobgoblins to 'fight by pairs'...allowing one or two to break contact, quaff a CLW and continue the battle.
(3) If they can break contact, they retreat through a rather flimsy door (fairly easy break DC), if not...they die in place. The next corridor is straight, with a slight up slope. The door is designed to slow the PCs down while the guards further back prepare the next line of defense...a large vat of boiled animal lard mixed with oil. They spill this down the sloped corridor, followed by burning bundles of oil-soaked hemp or other smokey, slow-burning material.
Remember that slight breeze? The outpost is constructed so that air is 'drawn' back towards the entrance. Choking smoke is pulled past the PCs - obscuring vision and making it difficult to breathe while the hobgoblins fire crossbows into their midst. The mixed flaming oil and animal lard makes for slippery footing (ala grease spell).
(4) All of this is designed to slow the PCs down while the main defenses can be prepared. Depending on the size of the outpost, the main defense could be a central chamber with murder holes, portcullises, balconies or other devices designed to give crossbow, long spear or pole-arm wielding hobgoblins "stand off" from the PCs and allow them to whittle the PCs down. Another fun thing to do would be to drop a concealed portcullis down after the barbarian and/or cleric have passed - to physically separate him from the other PCs, then use a secret passage to send a hit team against the 'softer' targets while the majority of the hobgoblin warriors dog-pile the barbarian. 'Aid Other' is definitely your friend here.
Playing dead, false surrender, hit and run, fight by pairs, giving up space for time to prepare a trap/all out defense, sending runner to get reinforcements, using non-lethal traps or props (such as a portcullis) to divide the battlefield and allow the hobgoblins to gang up on individual party members, concealed doors and passages to allow for flanking/rear attacks, physical separation with balconies, murder holes or other barriers that allow for missile fire or reach weapons use while staying out of 5' melee range...all kinds of fun stuff

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My 2 coppers...
~ Old One