Please bear in mind that I am not “interpreting” your dream for you. All symbolism in a dream is related to the dreamer. You created those symbols. You placed them where they were to help you understand a thing or situation that your “conscience” mind is having a hard time with. What I am pointing out are some of the more “usual” interpretations of these symbols.
“So last night, I had a dream that I was a knight living in the era of the crusades, when my castle was under attack”
Structures in a dream, a castle, a house, an office, can be viewed as the dreamers “person” Not just the body, but the personality as well. In this instance, you are viewing yourself as a “castle” that is under attack. What is a castle to you? Castles served many purposes, but were primarily a defensive military instllation.
“The villagers and the castle guard ran out and began defending the castle from the ramparts and walls,”
You begin your defense by drawing on internal resources, these resources do not go beyond the castle walls. This is, perhaps, something you need to justify to yourself before going any further in your defense.
“but the commander sent me and another group of knights outside the walls.”
If the castle is your body/persona, the “commander” is you, your “waking” self. Or a part of yourself that feels threatened and is compelling you to defend itself.
“We lined up behind a barricade of wooden spikes and aimed our pikes at the oncoming attackers.”
You are compelled to go out into “the world” to continue your defense once you have made certain that this “defense” is justified to yourself. But....you only go so far. Your “outer defenses” are well established and you do not go beyond these, that is, you remain on the defensive, rather than eschewing these to counter-attack the threat and “conquer it on it's own ground”
“I remember stabbing knight after knight with relative ease, taking down at least ten or so.”
You find defending yourself in your chosen manner to be very effective. Numerous attempts to breach your defenses are turned away with relative ease.
“What really caught my attention is that all the knights I was stabbing were female.”
Interpreting this is pretty much dependent on how you perceive “female” This is NOT limited to gender identification in the manner that our culture “dictates” to us. You the dreamer have made the “attackers” female. This could reflect an inner conflict just as readily as a conflict you may have with persons in The World that you perceive as “female” that is, holding a subordinate position in relation to yourself, irregardless of their biological gender. The “fact” that these females are also warriors is a telling clue.
“I felt terrible because they were just running into my pike for no apparent reason at all, dying by my hands.”
But they aren't dying by your hands, are they. You are doing nothing but defending yourself, and your attackers are destroying themselves on your defenses. Or so you perceive them to be. You feel bad about it nonetheless.
I don't know if you saw my sign “Psychiatric Help, Five cents” Just drop a nickle in the coffee can on my desk.
