Complex sentence: Construction began in 1565 using masons brought from Stirling; although notionally built for Carswell's patron, the Earl of Argyll, he intended it as a personal residence for himself.
Split into simpler sentences: Construction began in 1565 using masons brought from Stirling. Although the castle was notionally built for Carswell's patron, the Earl of Argyll, he intended it as a personal residence for himself.