I was trying to think how to research this

It depends on the person. If the king is a hands on guy, he might take charge and direct his forces. If he's not, he might hide in the cellar with the women and let the real men fight.

You mean in modern times? Medieval times? When? Also where? When you are the king, you can transmit orders however you please. Also, medieval castles (you know, the times of swords and shields) were mostly not as large as you probably think. The average medieval lord's castle was 2-3 rooms and they were lucky if they had an outer wall to protect the surrounding area. The lord himself would undoubtedly be the main person defending the castle. The king was more or less the biggest baddest lord in the region- the one to whom other lords owed their fiefdom. If the king's castle was under attack, the king himself would be the active leader of the defense. If one of the king's lord's castles was under attack, the king would plan either to allow the lord to defend the castle himself or perhaps send an army to attempt to drive off the invaders. Moving toward the modern era, castles became much bigger, but they still aren't anything like in the movies. A more modern king would likely delegate more power to his generals or viscounts and be less active in planning and participating in battle.

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If the castle were under attack, the king would be leading the fight to defend it. Contrary to the pansy-ass leaders of today who start fights and expect someone else to do the dirty work, royalty actually rolled up their sleeves and led the charge. This is one reason kings of old have always been respected throughout history as opposed to "elected" leaders who hide and protect their families when the fighting starts.

I suppose you want to know about a typical pre-gunpowder situation, and I expect you are concerned specifically with a European feudal-era castle. *** In the European feudal era, a king typically had authority over several castles in his kingdom, and often owned more than one personally. (There were, of course, many exceptions, and over-simply the farther back in time you go the fewer castles a king was likely to own). With regard to castles NOT owned by the king: they would have been owned by other members of the nobility. Typically each of those castles would have been operated by a member of the nobility who would have been recognized as "the Lord of the castle". (In some cases a seneschal would oversee the operation of the castle, especially while the Lord was absent. The seneschal was sort of a "vice-Lord of the castle". Sometimes a wife acted as seneschal, but more typically a male member of the nobility). Typically a noble who owned multiple castles would assign his underling family members (all members of the noble class) to be "Lord" or "seneschal" of his "secondary" castles, while the owner would have a "primary residence" or two. *** In MOST cases the Lord of the castle was also the owner of the land and the castle. (The primary characteristic of nobility is that they were land-owners, with enough arable land that they needed serfs to farm it.) *** So: the Lord of the castle (whether king or not) would typically be in a position where he could overlook the situation. ***Since such castles were intentionally without outer windows*** for defensive reasons, the Lord and his chief military officers would likely station themselves on the battlements (for the Lord, on top of the highest wall) in order to overlook the scene and analyze the situation. This would also give them a good vantage point from which to shout orders...though of course with a very large castle other methods, such as flag communications (semaphore) or (at night) torch-waving communication methods might be used. Of course no military officer can be everywhere at once, and so (of course) the officers in charge of more men or area than they could easily view would have messengers - scouts, so to speak, or message-runners, or aides, or whatever you would like to call them - to "run" messages to the various officers overseeing the battle at other locations in the castle. This often in addition to semaphore. Why in addition? Because semaphore could be read by the enemy as easily as by the defenders. A verbally-delivered message was unavailable to the enemy. SO FOR EXAMPLE: the Lord might be on the West Wall, and and officer on the East Wall sends a messenger to the Lord that the enemy has begun scaling the East Wall and is likely to be successful within the hour. The Lord can send a runner in turn to an officer who has a force that is waiting to act as support, and can by verbal message direct that officer to bring his troops to assist the officer in charge of the East Wall until the scaling action is repulsed. Meanwhile the Lord is still directing the action at the West Wall, or perhaps allowing his West Wall officer to direct the action while the Lord offers military advice. *** As one answer indicated, the king - and, indeed, all of the male nobility - were trained in the art of war. There are many reasons for this. A few examples: The foremost being that it took years and years of practice to become adept with various military weapons. It likewise took years to learn the ins and outs of military strategy. Armor (especially the late full plate armor suit that is common in our modern knight depictions) was EXTRAORDINARILY expensive. Only the nobility had the leisure and the finances to pursue a military career, and since the male nobility were also the head military officers it was typically mandatory (and quite beneficial for their continued high status and longevity) that they become adept in military matters. *** But the bottom line is that the method of transmitting orders would have depended on the size of the castle, on who was overseeing military operations at the moment, on the officers, and on the technology or techniques available to these people.