So I spent the last couple days scouring the Internet for rules and resources for player-built homes, strongholds, fortresses, and temples. And here's what I found so far (sorted by price, in ascending order):
Smitty's Modular Strongholds (PDF, $4.95 on DM's Guild). "A compendium of house rules for building and maintaining strongholds for DMs and Players of 5E Dungeons & Dragons, including: construction materials, costs, and time; fortifications, hirelings and NPCs, siege equipment and combat, and new feats and gear."
The Book of Strongholds & Dynasties (PDF, $5.95 on DriveThruRPG). "The first in the new "classic play" series from Mongoose Publishing.
Here you will find not only the lore of castles, but dozens of other strongholds and other buildings, enough to build all the major features of a city. Everything from the humblest farmhouse to the mightiest palace is covered between these covers. Having harvested the resources or had them delivered to you by professional builders’ guilds characters can build anything from a log cabin of wood to a towering fortress in crystal. Rules found herein cover the transformation of an ordinary manor house into a fortified tower or the skull of a dead god into a homely retreat.
Here you will find not only the lore of castles, but dozens of other strongholds and other buildings, enough to build all the major features of a city. Everything from the humblest farmhouse to the mightiest palace is covered between these covers. The Book of Strongholds and Dynasties unveils the Open Governmental System, a set of game mechanics that allows players to establish themselves as kings, princes, potentates, dictators, generals, theocrats and governors. After they have made their fortune as an adventurer, they can now try their hand as a monarch or as a politician. As a despot, governing with force, a plutocrat using your wealth to buy your way to power or a High King uniting the tribal warlords of a battle-ravaged country with nothing but your powers of leadership they must learn to wield new kinds of power.
An expanded and updated version of the Open Mass Combat System version II, completes this tome of architectural lore. After all, what use are rules for building strongholds without some rules to smash them down with!"
Ultimate Strongholds (5E) (PDF, $9.99 on DriveThruRPG). "Ultimate Strongholds brings you an awesome array of fantastic fortifications for your 5th Edition Campaign! This exhaustive encyclopedia integrates all levels of 5E play, from classic party-level adventuring to an easy system for creating buildings of your own, on up to the expansive kingdom-building and management rules in Kingdoms from Legendary Games! Explore tons of new options for buildings, rooms, augmentations, and fortified buildings, as well as unusual building locations. Dive into siege warfare with an array of siege weapons, ammunition, and special structures and building materials. Ultimate Strongholds goes far beyond the basic building blocks, of course, with a wealth of information for buildings integrated with kingdom and settlement attributes and a host of exotic strongholds and structures, from cloud castles to water walls, flesh piles to crystal palaces, hedge forts to ghostly redoubts, with rules for damaging such bizarre and magical constructions and the hazards they bring! Plus, you get over two dozen new spells ideal for defending your stronghold or breaching the holds of your enemies like battering bolt, deathless defenders, and spiritual pike wall, along with spells to enhance the fortress itself like elemental architecture and haunted ruin! Last but not least, you also get the brand-new castellan arcane tradition that guides and guards a fortress like no one else can!
The Legendary Games tradition is to combine rich story and background, innovative layout, beautiful aesthetics, and excellence in design that is second to none. This product is the latest in that tradition, and we hope you enjoy using it as much as we enjoyed making it. Grab this amazing 42-page 5E supplement by Ben Walklate and Jason Nelson and Make Your Game Legendary!"
Walrock's Fortresses, Temples, & Strongholds (PDF, $11.95 on DM's Guild) "The world is a wild and scary place, full of monsters, malevolence, and myriad evils to be expunged. Wouldn't it be wonderful to have a place to unwind? A place to hang your hat, build and customize, and to call your home? Wouldn't it be nice to leave your mark in the world? To build a permanent edifice as a testament to the ages, a bulwark of safety against the ravages of time? Would you build a mighty temple to your gods, proclaiming your faith far and wide? A dreary dungeon complete with a grotto of poisonous herbs and holding cells for those that would oppose you? Or a quiet cottage in the forest, an idyllic and relaxing retreat holding a simple magical study?
The Fortresses, Temples, & Strongholds ruleset holds options to build all these and more, allowing players to customize the world they live in and create a known presence that NPCs such as merchants, hirelings, soldiers, and spellcasters can flock to and partake in. A stronghold can become the center of a campaign, a place of respite, or a valiant bastion against the dark forces that threaten the world."
Strongholds & Followers (PDF, $30.00 at mcdmproductions.com). "
Strongholds & Followers is a supplement for 5th Edition that gives your character something to spend their money on and extend their influence in the local area. Raise armies! Research spells! Spy on your enemies! More than just a set of rules and charts, this book also describes a style of play that assumes your character becomes more interested in influencing the world around them. You’ll still adventure and fight monsters, but this supplement gives you tons of fun things to do during your downtime.
It only takes one character building a stronghold to radically change the nature of a campaign and introduce new narrative opportunities for GM and players alike! Huge new story opportunities arise! This book includes tons of examples for GMs to inspire them."
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If anyone knows of any other resources out there, drop a link!