Not saying its a good thing and we could certainly live without it but if you're talking about the 'essence' then that is what comes to my mind.
The Eiffel Tower.
Not necesarily the 'essence' of but I'd say the most important invention for the development of engineering (and architecture) would be the cmaera. Its helped spread ideas across continents and generations.
The Watts Governor. The key to good engineering is devising a solution that looks good, where the concept that is simple to understand, and that lasts the test of time. The Watts Governor is attached to the output of an engine. As its speed increases the balls are forced outwards under centripetal acceleration and this lifts the arm that closes the valve controlling the steam into the pistons. Less steam means less force from the pistons and the shaft slows down. Conversely if the engine slows down too much the balls can close inwards opening the valves to allow more steam to pass.
It was in use for well over 100 years. It was one of the first integrating control systems and was common place on most moving devices. It allowed the ouputs of machines to be regulated, which enabled them to be used more easily to create machines that could reliably drive the industrial revolution. It is such an iconic shape classically associated with engines of the industrial revoltion.
John Harrison, Price & Myers
The living bridges in India, amazing human manipulation of nature to engineer passage without any man made materials.