The military commander of a castle was called a castellan. The castellan might also be the majordomo, running the civilian staff as well. In French the same position was called a Chatelain. In German the same position was called a Burgmann.
If the question is who might otherwise posses a castle rather than a king or queen, then there would have been middle rank nobles who had a castle of some sort. While we tend to think of castles as elaborate stone structures, many were simpler. A more basic castle might be a simple stone keep with a surrounding wall, either of stone or or wood. Another basic castle style was the shell keep, which is a round stone wall, with wooden or stone structures built backing to the exterior wall, and with an open central courtyard.
it was the queen
In the local areas the castle.
it had draw bridges and cannons
the average size was 450,000sq. feet Castles vary immensely in size& importance. Krak des Chevaliers, in Syria for example, is huge. Others were little more than fortified farmhouses. I don't think there is a set pattern or size.
Generally, yes. Remember that the knight or duke who owns the castle is generally also the local law enforcement authority; the "dungeon" is also the jail.
When the King is away no one would look after the castle
Medieval Jesters traveled from castle to castle, in the Medieval town.
Kings and Queens did live in castles but not all castles had a king or queen living in them.
Castle
it was the queen
Protection
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The motte and bailey castle and the concentric castle.
In the medieval times, they used moats as a defence against intruders.
1066 onwards
In the local areas the castle.
a candle