A seat of the shadow government of Mechanicsburg, and an important story locale.
Carson von Mekkhan (nominally for Agatha's benefit, but we know it's really for our own) how this innocuous-looking place came to be so significant. As Seneschal to House Heterodyne in the Good Old Days, part of his job entailed looking after the running of the town of Mechanicsburg. In the chaos following The Other's attack, routine governance went out the window, along with a lot of other things. Eventually Baron Klaus Wulfenbach appeared and took over, installing one Burgermeister Zuken and a town council to be in charge, but Herr von Mekkhan wasn't about to let them do anything important. An arrangement was worked out: the council would continue to "report" to Klaus, while Carson (and eventually his grandson Vanamonde) would continue to pull the strings in the background, just as the von Mekkhans always had.
And that's where the coffee shop comes in. When we first see it, Van is Krosp I, Zeetha, Daughter of Chump, and Ardsley Wooster. Following an exchange that makes it clear that Van has been keeping his eyes open (and also that he's skeptical about Agatha, reasonably enough), he begins to play the host, and offers Agatha her first taste of coffee -- which can reasonably be described as In the whirlwind sequence that follows, Agatha establishes her credentials as a Spark by rebuilding Van's prized coffee engine, Spark Roast coffee is born, and Agatha gains a following .drinking coffee, as usual, as his grandpa enters with a band of mysterious outsiders -- Agatha,
The grapevine (and extended shadow government of Mechanicsburg) being what it is, events in the coffee shop eventually come to the notice of Gilgamesh Wulfenbach, who's in town to make sure his father (rather the worse for wear following the dust-up in Sturmhalten) is being well cared for at the hospital. He sends Captain Vole to investigate, which he does in his usual thuggish style, only to discover that Spark Roast hath to calm the savage Jägermonster, or whatever Vole considers himself these days. (Well, Spark Roast plus a blow upside the head with a great big wrench.) He's followed by a band of armed guards, but the townsfolk take care of them, while Agatha and Carson figure that they'd better get out while the getting is good and go fix Castle Heterodyne.
So in general, this isn't exactly your garden-variety coffeehouse. Stay tuned to see if more fun stuff happens here after Agatha gets done with the Castle.