In the alternate history steampunk world depicted in April and the Extraordinary World the (Second?) French Empire is more powerful and longer lasting than the one that existed in the non-fictional timeline. This is apparently largely due to the French Empire avoiding a conflict with Prussia as a side-effect of the untimely explosive death of Napoleon III during the film's introduction. This is consistent with the work's theme of a world frozen technologically and politically in many ways.

One of the many other side-effects of a resurgent French Empire is the appearance of a second Eiffel Tower in Paris, right next to and identical to the real Eiffel Tower of history.

Is there some reason given in the film or hinted at in history that explains why the second Eiffel Tower was built?

At the end of the movie, we learn that the launch pad for the Lizards' spaceborne missile is deep beneath the second Eiffel Tower and the second tower is destroyed during the launch of the missile, symbolizing a return to normal political and technological development. But it doesn't appear to explain why or how the second Eiffel Tower was built or what the reasoning behind it is (unless its purely symbolic).

Why are there two Eiffel Towers?