In Underwater Cities, which lasts approximately 30-45 minutes per player, players represent the most powerful brains in the world, brains nominated from the Earth's overpopulation to create the best possible and most livable underwater areas.

The main principle of the game is placing cards. Three colored cards are placed along the edge of the main board in 3 x 5 boxes, which are also colored. Ideally, players can place cards in slots of the same color. Then they can take both actions and benefits: the action depicted in the slot on the main board and also the benefit of the card. Actions and benefits allow players to absorb resources; building and upgrading city domes, tunnels and production buildings such as farms, desalination equipment and laboratories in their personal underwater area; to move their marker on the initiative track (which is important for player order on the next turn); to activate the player's "A cards"; and to collect cards, both special and basic cards that provide better decision-making options during play.

All almost 220 cards - whether special or simple - are divided into five types depending on the way and time of use. Underwater areas are planned to be double-sided, giving players many opportunities to gain VPs and ultimately win.