The trapdoors are similar to the one you saw at the entrance of the Citadel. They open downwards when someone or something walks onto them. They have been propped open by spikes, as to not let the mechanism close again.

OOC: So just to clarify the map according to our plan - so Spotty is up at the top of the long corridor. Gwen, Martin and Hef would be in that bottom left room at the north door waiting for Zak, Spotty and Renny to come in that first cracked open door on the left in the big hallway. 

As the party goes their ways, Zak assumes the plan is in action and will open/head into the first door/room on the left of that long hallway hoping that that room hooks up with the room that G, M and & H are about the barge into. 

P.S. Zak doesn't care at all where Meepo is :)

(To make things quicker, I’ll assume you peek into all the little cells)

At first glance, they just look like abandoned cells. Three of them seem to have become rat nests, most likely the rats whose corpses are now decorating the hallway floor.

Map (from phone):

 PS: Meepo forgot his weapons next to his bedroll, and seems too frightened to do anything anyways. 

Since Renny is trailing behind, she does take a few moments to peek into the cells just to see if anything interesting catches her eye. She'll use the stone she'd be keeping with the light spell to help light the rooms.

Perception 13

"Shoot... that door didn't connect to the other room like we thought. Let's go up further then to that larger room with the open trap doors."

Zak, as usual, without regard for his safety, still remembering their original plan to come at the goblins from 2 directions, will try and open the west door in that room at the end of the hallway with the fountain. Zak sees the trapdoors propped open and thinks to himself "there's no way that this door is trapped with those other traps in this room."

He also has no desire to check out the fountain or anything else in this room for that matter. His head is in the plan to surround the goblins that he heard (thinks he heard...) earlier. 

"Sure. Just watch your step," Spotty says as he moves toward the fountain, curiously inspecting it.

Perception: 22
Investigation: 23

Spotty:

Zak: As you open the door, an oppressive smell suffuses the air, rising from chewed carcasses of cave rats, smaller vermin, and a few humanoid-looking creatures. The bodies lie upon a floor of filth, old bones, hair, and fur that combine to make a particularly large and vile nest. Three giant rats occupy the area and seem to be feasting on a somewhat fresh body, with a monstrously swollen female rat nesting along the northern wall.

You are facing 4 Giant Rats (including one monstrous female) in the room ahead.

Rats' Initiative: 13

ROLL FOR INITIATIVE !
(For speed, let's say that the rats are surprised at Zak's sudden entry, and you can all take a first turn already)

Encounter Map:

Renny:

Initiative 15

Renny probably hears something from Zak or Spotty to get her attention. She moves across toward Spotty (D8) and the first thing she sees is the big mamma rat. She hurts the Fire Bolt she was already preparing toward it.

Attack: 9 Damage: 9
(if this is a miss, is there something near mamma rat that would be set on fire instead?)

She looks back down the hall toward Gwenant and says, hopefully loud enough to be heard but not too loud, "Bigger rats!"

Initiative: 14

Gwenant is idly standing by and quietly counting down until she thinks the others will be in position for the ambush when she hears Renny call out. She jerks up, confused, then shoots a concerned look at her companions and dashes to where Zak is at the door.

((Not sure if we back-line folks get the surprise round, so she'll dash either to a tile next to Zak if she is able to reach him or to the nearest rat if they escape the room))

Spotty's initiative: 12

(so, do I get 2 turns? :p)

"Oh, boy. More rats." Spotty quickly nocks an arrow and sends it flying at the biggest one.

Attack: 7 - 10 damage

Renny: your bolt misses, and doesn’t really set a huge fire, but a few bits of wood and clothing around the big rat start smoking, it is somewhat humid around.

Gwenant: you reach Zak.

Spotty: your shot completely misfires and drops into one of the trapdoors. And yes, you would get to take another turn before the rats. I’m officially going with the rats being completely unprepared for Zak’s sudden barging entry.

Those with a view on him notice Meepo jumps into the closest cell and hides.

Spotty grumbles to himself and takes another shot.

Attack: 7 - 9 damage

He misses horribly, again. "Mom would be so disappointed..."

Zak is more disappointed that the room didn’t link up with the room he heard the goblins in where the others are then about the pesky rats. Annoyed. He lets out a frustrated grunt and then runs at momma rat with his greatsword trying to stab into her body. Run to A3.

Initiative 14

Attack: 6 Damage: 6

Renny takes a moment to look at Spotty sympathetically, "We both missed."

Initiative: 7

As he ambles to join the rest with Gwenant, Hef's ears seem to perk as he hears a door creak in the distance, "Oh no..." he mutters as he hustles down the hallway.

(Let's assume Martin also approaches during the surprise round. And Hef if you act on Round 1 before the rats, and position yourself near Gwenant or anywhere in the chamber, you'll attract the second rat attacking Gwenant, taking that attack instead of her. It misses anyway.)

Hef, Martin and Gwenant run up towards the second fountain chamber. Renny fires a fire bolt and misses, followed by a missed arrow from Spotty. That doorway is just too narrow. Zak jumps onto the biggest rat there, the mother rat, and stabs her hard, but finds there is a lot of fat and gross pus coming out of her body, and she still has some fight in her.

Round 1:

Spotty fires again, and misses again. Definitely a small door. Mama rat and one of the others bite at Zak's legs:

Mama rat: Attack: 21 Damage: 4
B3 rat: Attack: 19 Damage: 4

Zak, please make a Constitution check on your next turn.

The other two can smell more creatures around and run up to the door, stopping to bite at Gwenant:

B6 rat: Attack: 22 Damage: 4
C5 rat: Attack: 12 Damage: 4

Initiative:

Spotty/Hef -> Rats -> Rest of Party

Encounter Map:

Spotty quickly nocks and fires another arrow, seeming more focused now that Gwenant is in melee.

Attack B6 with advantage (pack tactics): 9 13 - 9 damage (sneak attack)

Hef closes the distance and attempts to end the rat that's made it into the room.

Attack: 18 Damage: 8
Bonus Attack: 8 Damage: 6

Gwenant grits her teeth as she suffers yet another rat bite. These things clearly aren't normal... even accounting for their unusual size! She jabs her trident at rat C5, hoping to clear a path into the room and aid Zak.

Attack: 18 Damage: 6

((If both rats next to Gwenant are dead/no longer next to her, she'll move to C3))