The first fortifications at Pau date from the 11th Century, and the hilltop location was the seat of bishops, counts and viscounts. There were (are) dungeons, and an early legislative body like a house of lords perhaps convened here.
In 1553, Henry IV was born here. He was an heir but not established as the sole legitimate heir -- others had claims-- to the throne. It was an era of wars of religion between Catholics and Protestants that continued to fester, explode, get stoked by king and others alike, fester, explode and reach terrible heights in the St. Bartholomew's Day massacre, hunting and killing Huguenots as heretics. Henry II himself had been a Calvinist, then became Catholic; among other affiliations. See http://henri-iv.culture.fr/en/uc/01_00-A%20prince%20is%20born.
Pau, Chateau. Medallion, Henri IV. France
The conflicts: The Huguenot Protestants were set up as the villains in many occurrences, including and blamed for disruptions during his own first wedding festivities, see http://henri-iv.culture.fr/en/uc/01_01_00-The%20marriage. Placing the claims and conflicts in context is not simple. The older King Henry III of France was King of Poland for the year or more before he became King of France in 1574-1589. In 1589, he affirmed Henry IV as his true heir. Good King Henry (IV) then became King of Navarre 1572-1610, but as Henri III in the annals there; and King of France (the first Bourbon monarch) 1589-1610 -- and then he was assassinated.