After a quick detour to cover a bit about how a sandbox setting works during actual play in part 2, now I want to get into how I run a sandbox cape-and-sword campaign.
First, compile an Appendix N for your sandbox which lays out the genre influences for the campaign.
Roleplaying games are traditionally composed from a farrago of sources and influences, and so are their settings, both explicit and implied. A discrete list of books, comic books, movies, artwork, and other media provides a focus for preparing elements of the sandbox which fit the genre. A sandbox which reflects its genre influences may provide the players an immediate familiarity with and an intuitive feel for the game-world.
The less familiar and more remote the genre influences, on the other hand, the more the players must learn about the sandbox before being confidant in acting on their understanding of the game-world. This is why clichés and archetypes abound in roleplaying games and other media - they paint a vivid picture in the imagination which short circuits the need for lengthy exposition. Increasing the ease with which players can choose a reasonable course of action for their characters helps facilitate sandbox play.
The significance of a sandbox's genre influences also holds for genre-mashups; if the mashup involves two immediately accessible genres - swashbucklers versus Cthulhoid horrors in Revolutionary France! - then the players may gain the same benefits of familiarity as a single genre. On the other hand, a burgeoning excess of hetergeneity may result in incongruity, an invidious sense of mission creep - I still hear players complain about the big setting reveal in 7th Sea, for example.
Now cape-and-sword genre tropes are far from universal - a subject I will get into at length in future posts - so choosing the sandbox's Appendix N helps narrow the field a bit. A setting appropriate to The Princess Bride may be may look very different from one appropriate to Captain Alatriste. The specific genre influences particular to a given sandbox help to give it a shape more readily recognizible to the players.
The Appendix N for my Flashing Blades campaign lists my main sources of inspiration. The story which first prompted me to run Flashing Blades was Robert E. Howard's "The Shadow of the Vulture," set at the 1529 siege of Vienna; I subsequently drew from - naturally - REH's pirate stories, "Swords of the Red Brotherhood" and "Black Vulmea's Vengeance," as well. The other sources that make up my Appendix N build on this.
Melodramas like The Prisoner of Zenda and Swordspoint are absent from my sandbox's Appendix N, though I really like both stories and their sequels. Comic movies like Swashbuckler and the unintentionally funny The Musketeer are missing as well, as most of the humor in the campaign is found, not deliberate or satirical.
Having a firm grasp of the genre and associated tropes gives me a framework on which to build the setting elements of my swashbuckling sandbox. The pulp tales and related sources which inspire my campaign contain rough, flawed anti-heroes and cruel, complex antagonists, and the non-player characters populating my sandbox reflect this - rock-jawed champions and mustache-twirling villains need not apply - as do their agendas, which mostly revolve around the pursuit of fortune and glory and the power that comes with both.
Random encounters play a very important role in my sandboxes. They represent the living setting, a nexus between the travels and actions of the adventurers and in media res events which in some instances follow from the agendas of significant non-player characters in the campaign. Many of my random encounters feature named npcs for this reason; they provide an opportunity for the adventurers to meet and interact with these characters, to build a relationship - for better or worse - with the movers and shakers of the setting. From my Appendix N sources, I can draw ideas for the kinds of encounters representing the genre tropes in those stories. For example, the adventurers in my sandbox aren't likely to hear of a merchant looking to hire caravan guards, but they may be approached by a gambler or a moneylender looking for swordsmen to help recover a debt.
Now that gambler or moneylender will rarely exist in isolation, and in part 4 - part 4?! - I'll get into social relationships as a setting map in my swashbuckler's sandbox.