Hi everyone.  I need your help.

My party has left the Antarctic where they were all born and had a harrowing voyage North.  After sinking, they drifted to a tropical, mountainous island.  Much of their gear was lost in these mishaps.  I want to write a dungeon (3-8 hours of playing time) on the island.  I am looking for a pretty traditional hack and slash dungeon but it needs to make sense and be logical.  They also need opportunities to re-gear.  Lastly, they need a way off the island onto the mainland to continue the campaign, likely going to a ruined castle destroyed by a lich hundreds of years ago, with a stop in a town beforehand.  

I am thinking the island used to be a thriving port but was destroyed hundreds of years ago.  The port is in ruins but there are stone docks with ruined ships on the sea floor (new gear if they are brave enough to explore the wrecks).  Skeletons roam the docks.  They have no magic weapons left so they will have to be clever in how to kill them.  There are guard towers scattered over the mountainous island, several overlooking the port.  All have signal mirrors and ballistas on the top level, although some of the gear will need to be repaired to become operational.  The ballistas function as +1 weapons and can be used to snipe off the skeletons.   

There is a central keep on the highest point.  I need to populate the upper levels with some type of monster(s).  There is a gate or portcullis in the lower levels (to keep out the undead and beasts) and pirates are still using the dungeons as a staging area.  The pirates have tunnels leading to a grotto, where they store their ship(s).  The party could steal, stow away or take a boat/ship to get off the island.  

Alternatively, there are black roses in a ring on one of the hills, surrounded by stone cairns.  These are graves of soldiers and leaders who died defending the island against the lich who attacked.  This would explain why the island is now uninhabited and there may be some sort of journal or log to give them more information.  If they plant the black roses in front of a gate, it becomes a portal, taking them to the mainland.  

I like the idea of their being multiple ways off the island.  

Other ideas?  I really need help populating the tower in a logical way.   What creatures would steer clear of the pirates or not be able to bother them?  Ideally, creatures who are not interested in treasure or weapons so there would be some gear and rewards for my players.

The novel “The Barsoom Project” may be of value in some ways, as might its predecessor, Dream Park.

Barsoom uses an “Inuit” based frozen adventure and Dream Park uses a tropical Cargo Cult based one.

i suggest both because they may help since you can pull elements from both.

strange tunnels in the side of a volcano, some sort of well-guarded society inside with outposts connected by swooping railcar paths. if they announce themselves then they're captured roughly only to be soon released as guests... before being hunted for sport. if they stay hidden they could steal uniforms and weapons. they'll come across scars of battle, encounter crippled and failed constructs, and discover a plot to send an improved construct out to terrorize a city. this could all be perpetrated by a sorcerer, a masked lich, or three kobolds in a robe angry for being marginalized even as the wizards of the city grew famous and fat. or maybe it's a druid who has bent the people's favor against arcane magic to have it slowly outlawed only to swoop in and "protect" them when no one else can.

so... "other ideas?" yes. Nomanisan Island from the first The Incredibles movie.

TV show "Lost" is one of my go to's. Also the Isle of Dread adventure from 2E was revised for 5E by a third party publisher (I don't remember which).

I am a fan of using Sly Flourish's method for setting the stage on adventures and letting the players bite on what sounds interesting to them and their characters. One of the easiest ways to do this is to lay out five or six "fantastic locations" so to speak. You invest some time and come up with two or three sentences to describe areas of your setting that you think might really catch the attention of the players and prompt them to investigate further. Feel free to write up as much as you feel motivated to write, but you only need two or three sentences.

Based on what you said, you could set your hooks with a few observations as the party arrives and has their initial look around:

• As you approach at the island, the thick blanket of tropical green trees and plants slowly reveals signs of civilization in the distance along the shore in the form of small stonework structures and a stone dock that pokes out into the water. As your boat pulls closer, the general silence and absence of movement around the docks, along with the overgrown and cracked stonework, speak to a town that has been long abandoned, though you cannot yet determine why.
• Gazing beneath the clear blue waters near the docks reveals the wreckage of ships below the surface in various states of decay. The ships vary in style and design, indicating a legacy of trade from far-off lands. One has to wonder what treasures and dangers they might still contain within their old collapsing structures.
• Disembarking from your vessel, the town spreads out further inland than you initially expected, with thick tropical vegetation overgrowing the streets and partially-collapsed storefronts exposing shadowy openings. A faint scent of rot carries on the salty breeze, and a perceptive listener notices an unnatural clicking every now and again. With some investigation, animated skeletons can be seen wearing rotting rags and carrying crude tools and weapons.
• A survey of the inland hills above the town indicates the presence of several overgrown stone towers positioned to overlook the town and docks. These buildings stand two or three stories high with windows in the walls and broad turrets on top. If the party can find a way to them and get inside, they look like the best options for a defensible shelter.
• High atop the highest point on the island several miles away, you can see the structure of a large stone building, perhaps an old keep where this society's leader lived. Getting to it through the thick jungle will not be easy, but it would be worth the risk and effort. An unfamiliar flag still flies atop the highest tower. Could it still be inhabited by someone or something?

Now that you have a few tempting locations to offer up to the players, come up with ten secrets and clues that can be revealed along the way. If you don't end up using all of these, that's fine, but it's nice to have a reference to fall back on to keep things interesting and to reward exploration:

• The largest ship beneath the waves is a galleon called "The Aegis" and it was carrying a gift to the lord of this island from a nearby kingdom on a mission of partnership. This gift can still be found deep within the ship's hold in a well-made locked chest.
• The ruined port town has two dozen skeletons who wander aimlessly. They were townsfolk who were killed in the lich attack and resurrected into undead minions.
• The pirates know to avoid the town after dark, when the skeletons are more active.
• The skeletons' eyes glow red during the full moon, and they become faster and stronger until the following dawn, though nobody knows why.
• There are six guard towers on the hills overlooking the town. Most are overgrown and abandoned, though three of them have ballistas on the turrets that can be repaired with time and effort.
• Tropical fruit grows throughout the jungle, providing sustenance to those who consume it, though people should avoid the mysterious fruit of the blue flowering tree, which can send someone into a long deep sleep, or worse.
• The pirates who inhabit the castle on the highest hill number twenty and are led by a lizardfolk captain. She is the only one who can read the books written in the language of those who lived on the island before, and only she knows about the prophecy and the attack that brought the civilization to an end.
• An old cave leads from the castle down to a seaside grotto where the pirates have two ships moored out of sight and away from the elements.
• In two weeks, a powerful storm will blow into the island, making sea travel impossible for a week and destroying any ships left out in the open.
• Black roses planted near stone cairns on the back of the island mark the graves of those who died defending against the lich attack, the roses have magical properties, and if replanted near the gate, create a magical portal to the mainland. This is written by a sage in the books among the castle, but in a long-forgotten language.

OK, so with that handled, just come up with a handful of potential encounters to fill your 3-8 hour session. These need not all be combat encounters, and even those are more interesting when they offer options to resolve them differently.

• Exploring the sunken ships in the harbor. Five ships ranging from a small skiff to a huge galleon. Sharks in the water (reef shark to giant shark, depending on the party's level) and some lacedons. Every ship should have something worth exploring, from useful, but mundane supplies, to a treasure hidden in the galleon.
• Exploring the port town leads to encounters with packs of skeletons, up to two dozen in total. If the party remains in town after dark, the skeletons become stronger. The ruins can give clues to the lich attack in the form of fireball-scorched stonework, evidence of ritual killings, or perhaps even an old shrine to a profane deity.
• Getting to the guard towers or the castle requires traversing the jungle. Exotic plants and beasts roam the underbrush. Paths can be located with differing levels of difficulty leading to the towers, the castle, the grave cairns on the back of the island, and even to the hidden grotto on the coast.
• Pirates in the castle will likely be hostile to outsiders, though the lizardfolk captain is more pragmatic and can be tempted to parley with the party if there is good reason.

With all of that in hand, start offering up one or two fantastic locations and see what they bite on.  Good luck!

A few other places where you might look to other D&D material for inspiration on your island adventure concepts include the Ghosts of Saltmarsh and the Explorer's Guide to Wildemount (specifically the islands along the Menagierie Coast). And of course Call of the Netherdeep has aquatic settings as well.

A harpy (flavored as sirens) could populate the shorelines in large numbers, if there was no human population to hunt them down - that would be enough to keep pirates away.

The banshee who is a restless spirit can either be a monster, or an NPC, to help provide a method for them to get eternal rest. As well as the other, traditional undead (ghouls, ghasts, ghosts, skeletons, zombies, etc).

You could have the "society" that exists be akin to grimlocks - where they're just privative, mindless, humans, who believe their old god (the Lich, perhaps?) still exists. Or some humans who are still there could be jackalwere. Perhaps cursed by the Lich, as they refused to bend the knee to him; and now have given in to their evil that was infused by the curse.

A Spectator who survived whatever happened to the island, there, to provide some information (lore, as needed). Similar near the mountains maybe some galeb duhr who could provide information about the island's history.

The classic darkmantle, stirge, piercer that live in the caves of the island. And probably large populations of them if the island is really untouched. Shriekers are fun and other monsters could use them as alarms.

A Manticore could have taken up residence on the island.

The various Blights (Twig, Vine, etc) that might populate the woods, perhaps given life by whatever happened to the island (corrupt magic). You could also have Ettercaps in the forests. Probably giant spiders too - again, probably large populations if there's been no humans or anything to diminish their numbers.

Depending on the level of the characters, hags might be interested in the island as well. Especially a green hag who felt whatever great travesty happened here.

Hello everyone.  These are really great ideas, first rate.  I plan on stealing most of them.  My only worry is I have three days to completely write it up and I am working full time again, giving me less time to write.  Again, many thanks!