Arstotzka has its own custom Border Guard unit, a cheaper version of the Rifleman that gets combat bonuses fighting on native soil. There's also the Border Checkpoint building, which adds protection from spies and produces extra gold each turn. Arstotzka begins the game with its custom "Glory to Arstotzka" trait, which increases worker production at the cost of population happiness and city growth speed. All cities also start with a free worker.
The only downside to this mod is that it requires Brave New World, the latest expansion for Sid Meier's Civilization 5, in order to play. Sid Meier's Civilization 5: The Complete Edition, along with several other games bearing the eponymous developer's name, are currently available in the Humble Sid Meier's Bundle.