I saw this sentence somewhere, and since I am not native speaker, this seemed weird to me.
The part "are many" made the sentence not complete in my eyes and made it weird. Did they use it correctly?
The techniques and tools we use to defend our castle are many, most of them not available to random people.
Please don't mind that the sentence does not make much sense in general.
I don't know why you think it sounds weird to you. But in any event, are here refers to techniques and tools, which is a combination of plural nouns and therefore are is the correct form of the verb.
Try this out instead:
There are many techniques and tools that we use to defend our castle. Most of them are not available to random people.
The word "many" in your sentence describes the group of "techniques and tools", telling the reader that there are many techniques and tools in that group. "Many" works as an adjective in the sentence.
Imagine replacing "many" by a different adjective, for example "secret". Then it reads:
The techniques and tools ... are secret, ...
The usage is just the same with "many", but is less common.
