
A wagon can hold 3 ships. Each ship can hold another wagon, allowing 3 wagons in the space of 1, and a further iteration puts 9 wagons in the space of 1. You can potentially cram infinite units inside a single space.
This can be safely hidden inside a building to avoid notice, but the most terrifying aspect is infinite movement speed. Each iteration allows you to move the speed of one more wagon further in a single turn, possibly dropping units off along the way.
Maybe we should exclude transport ships from entering wagons