It was advertised as a safe haven where people could survive the upcoming apocalypse. Only "perfect" people were allowed to live there. However, no person admitted to Sweetville left it or had any contact with others living.
In fact, Mrs Gillyflower planned to send up a rocket containing red leech venom, which would explode and shower the population with poison. The people of Sweetville could then be revived to repopulate the world. Madame Vastra and Jenny Flint removed the venom from this rocket before it could be launched. (TV: The Crimson Horror)