There's a group of mostly human adventurers, having an intense conversation with a group of equally armed tieflings on the wall. 

Human adventurer: OPEN THE GATE!

Tiefling: Zevlor ordered the gate closed, and where is the Druid?

Human: There's no time, the goblins are right on our tails!

Tiefling: WHY DID YOU BRING THE GOBLINS HERE?

On cue, the goblins arrived, and shoot the tiefling on the wall. 

The gate isn't getting opened any time soon. 

"Anyone have any strong opinions on which group of people we stab?"

"Goblins are very rarely the people you want to side with."

"Ugh, what if we just side with no one and move on with our lives?"

Cayden likes goblins fine, actually. They're dumb and impulsive but Cayden is also dumb and impulsive so he relates. He thinks that most goblin-nongoblin conflict is downstream of nongoblins seeing a goblin and instantly deciding to murder them without even considering the possibility of peaceful coexistence.

...This isn't really the point.

"Anyone know who this Zevlor guy is?"

"I think I heard of him as a figure of note in Elturel?" There is a brief moment of calculation, before his face falls.  "If we're near Elturel, we need to side with the tieflings."

Neither Lae'zel or Shadowheart recognise the name. 

"No, we really don't, actually. Look, these tieflings have a nice wall, which is a step up from what they had before."

The goblins and the adventurers are squaring off like they're waiting for the other to make the first move. 

"--Gale, can you give me the rundown really quickly?"

"The city got sent to the Hells. People blamed the tieflings, even though none of them had anything to do with that plot. Once the city was returned to its rightful plane, they were cast out.

These are refugees, Cayden."

A middle aged, armoured tiefling is up on the wall, giving aid to the person who was shot. 

Another human adventurer has appeared on the wall too.

"I'm in. Shadowheart, Lae'zel, Astarion, your choice."

"I see no reason not to wet my blade."

Shadowheart has decided that instead of verbally confirming, she is going to cast a spell over the party. 

Cayden feels some of his old accuracy return to his limbs.

"If you're going to all insist--"

The human adventurer on the wall has decided to a) no longer be on the wall and b) reopen hostilities.

He hits a goblin with a bolt of magic. "Provoke the Blade--" He jumps off the wall, slides down a scree slope, and draws his rapier with a flourish. "-- and suffer it's sting!"

Having a slogan is so cheesy.

How do all these assorted humans and goblins feel about an unexpected attack from the rear?

The humans and tieflings are happy!

The goblins are not happy that there is a variety of variously martial trained people fighting them. 

The most difficult part of warfare is unit cohesion, which is the fancy tactician word for "staying in fucking formation and doing what you're supposed to."The fundamental goal in most battles is to break down the enemy's unit cohesion and get them to rout, which is the fancy tactician word for "run like hell in the other direction." Once the enemy has routed, you can kill them at your leisure, although if you just wanted them not to be in front of the castle you might consider the rout to be good enough. 

This is why morale is the most important. It's much easier to get the enemy to rout if they're hungry and their shoes have holes and you have a reputation as the baddest motherfuckers around.

Cayden is less good than he used to be at killing things.

Cayden is exactly as good as he used to be at conveying that he's the baddest motherfucker around. 

Also, it's really not hard to convey that message to goblins. 

The goblins do not particularly want the gates. They'd be nice to have,  but they don't need them right now.

What they want, and what they have is their location. 

They are quite willing to retreat! It's not the most cohesive retreat, but they are certainly skedaddle away from Cayden and the several magic users.

Cayden continues to cartwheel his way through volleys of arrows, etc. until they are safely well away from the gates. Then he checks on his party members. 

Lae'zel is covered in blood, but it's fine,  it's mostly not hers.

Shadowheart seems to have picked up a hand injury in this process somehow? 

Gale and Astarion are both standing on an outcropping of rock they were using as high ground.

Excellent.

...Except before the goblins showed up there was a dispute about humans getting in the gates and he's a human who wants to get in the gates. Problematic. Maybe he can just stand around awkwardly until something happens to him. 

There's a pale curly haired human who seems to be the leader of the adventurer's. "Thanks for the assist, mate."

The two tieflings on the wall are now opening the gates.