Right outside of the main city center is a hotel. It's a decent sized hotel. The owner is an Arcadian sympathizer himself, so he has given them the top floors of the hotel as their base of operations as long as they don't scare off his guests or use up all of his wifi.

Because it's in a public building and there's the possibility of being discovered, everything in the base can be hidden or disguised as something else so it looks like normal hotel rooms. The walls flip around to reveal maps and screens, the tabletop comes off to show secret documents, the beds flip up into cabinets where the hold the weapons and costumes, and the tv...well, actually it's just a tv.

Everything is so well hidden and locked that it couldn't be found by accident or on purpose unless someone knows exactly where it is and knows how to open it...And has an Arcadian microchip in the heel of their hand.