What's the Kennection? #301

What castle, a UNESCO World Heritage Site in Seville, Spain, is the oldest royal palace still in use in Europe?

Which branch of mathematics was simultaneously developed by rivals Isaac Newton and Gottfried Leibniz three centuries ago?

The Scottish delicacy "Finnan haddie" is made by cold-smoking what Atlantic fish?

What name for an Australian river translates into Spanish as "nevada," giving us the name of a U.S. state and mountain range?

What pop band—a trio, actually—had their biggest hit with 1983's "Hold Me Now" before changing their name to Babble?

What's the "Kennection"?