It actually got some pretty decent reviews on Trip Advisor (and some bad ones but more good than bad from what I just read). I also noticed that the manager at Castle Inn responded to a lot of the good and bad reviews which tells me they do at least care. Maybe call the hotel directly and speak to the manager and tell him/her your worries. Seriously, it might mean they make sure you get a decent room since you are on the internet and they know you will likely post a review on here and other sites too. There must be some postives to the place to get the amount of good reviews they do receive. And its ranked about in the middle of all hotels in the Anaheim area.
All said, at this point, there is nothing you can do about it but try to relax and go in with an open mind. Take a look at your room before you unpack, check for bed bugs (which I do at any hotel I stay in, if you google bedbugs you can find info on how to check for them, they do leave telltale signs) and if the room is not good, ask to get another one.
I haven't stayed there, but our neighbors we have known for years go there every year. They swear by it and their kids for years thought the Castle Inn was the "
Disneyland Hotel" since it was a castle!
And just a note for future reference, if you call a hotel directly and tell them that you saw a low rate on a third party site, most hotels will match that directly through them saving you the no cancellation headache. I had this happen a few years ago when I booked a non cancellable room through Expedia and my son got swine flu. Expedia did nothing, but the hotel did cancel it for me and told me to call them directly next time. Which I have since done with several places and I always get a better rate than their internet rate.
I think you will be fine, you will be at Disneyland after all! Good luck, I think many of us understand how easy it is to worry over your hotel choice but in the end, it almost always works out just fine.