The Battle of Polytopia contains many confirmed Easter Eggs added over time in different updates. Easter Eggs are intentional, while bugs are not.
Nature Bunnies
In single-player, tapping an unoccupied tile in your territory ten times will spawn a Nature Bunny unit. Nature Bunnies have the Crush skill, enabling them to destroy any building and units on tiles they move to. Just like any other unit, Nature Bunnies can attack, kill, and be killed. Also, although Nature Bunnies can enter cities, they cannot capture cities.
Nature Bunnies cannot be spawned in multiplayer (including both Pass & Play and online).
Luxidoor Easter Egg
Even if you don’t own Luxidoor, there is still a 10% chance of Luxidoor appearing as an opponent in your (single-player) games. The official reason for this Easter Egg is that it is an “apology” for Luxidoor is more expensive than other tribes.
Note that you can force Luxidoor to spawn if you do not own them by playing a 1v1 game and disabling all the tribes you own (except the tribe you are playing).
Mixed Tribes Easter Egg
The Instagram post announcing the Mixed Tribes Easter Egg
If you have two tribes with three stars each, you can combine them by tapping and holding the icons for both tribes in the tribe selection screen.
The tribe you tap first determines the first four characters of the name, the city style, and the unit style of the mixed tribe. The tribe you tap second determines the last three characters of the name, the terrain style, and the technologies (including the starting and special technologies) of the mixed tribe.
Basically, the mixed tribe has the gameplay of the second tribe but some of the aesthetics of the first tribe.
Floating or Sinking City
An example of both the floating city and the Mixed Tribes Easter Eggs
Tapping the tiles on the bottom left and the bottom right of a city alternatively causes the city to float or sink. Tapping on the bottom left first causes the city to rise into the sky, while tapping on the bottom right first causes the city to sink into the ground.
This change is only cosmetic and does not prevent your city from being seiged.
Training a unit from the city or ending your turn reverts this change.
Sunrise
The Instagram post hinting at the Sunrise Easter Egg captioned, "Sometimes it takes up to 15 seconds to reach enlightenment."
Holding the starry night sky background for 15 seconds, then releasing, causes a "sunrise." The sky will change to a pinkish-yellow color similar to the main menu. Holding the background for 15 seconds again or relaunching the game reverts this change.
Other
Elyrion Language
If you own the Elyrion tribe, then one of the languages available in the settings will be the Elyrion language.
Multiplayer Friend Codes
If you try to add yourself as a friend, the following message appears:
Me, myself & I
Seems like you tried to add yourself as a friend. That might sound like a nice thing to do but it would add an existential layer to the game that we cannot handle at the moment. Please submit a player ID of someone else.
If you try to add a friend but submit an empty player ID, the following message appears:
Emptiness
The player ID you entered was complete empty! You should not try to make friends with the void, it is a very lonely path.