4 ideas
In 1469 Isabella secretly married Ferdinand of Aragon against the wishes of her half-brother Henry IV, under a marriage agreement that kept Castile's sovereignty in her hands. After she claimed the crown in 1474, the 1475 Concord of Segovia settled that she was Castile's proprietary monarch, with Ferdinand ruling beside her and not above her.
When Henry IV died in 1474, Isabella had herself proclaimed queen in Segovia without waiting for Ferdinand or for consensus. Her claim was challenged by Henry's supposed daughter Juana. Her reign shows a succession dispute being settled by the claimant who publicly takes the role and then defends it militarily, as in the War of the Castilian Succession, rather than by the one with the stronger legal case.
Isabella's program to build a unified Castile around one faith led directly to the Inquisition in 1478 and the expulsion of the Jews in 1492. It also financed the conquest of Granada and Columbus's voyage in that same year. Reading these as parts of one state-building project, rather than as separate successes and atrocities, shows how a drive for cohesion creates a need to define and purge those who don't fit.
The investment was modest compared to the potential return. Her reign shows that high-variance ventures get funded after a state has secured its core position, when the downside is small in relative terms and there are surplus capacity and ambition to spend.