I'd say any basic building tub (3303, IIRC, is the classic "building starter tub") I think is an "essential" to anybody who likes to build.
I absolutely agree and I'd say multiple tubs. Because every single project I start, I end up having to get more basic bricks. Right now it is 1 by plates. I think we over look the need for and focus on the color we choose as our favorite..most castle people would say one of the greys or all (me that's tan) but then when it is time to build land...whoa...I have no green, or blue for water...and oh my what about brown? I need basic dirt. Basic brick is essential.
Then if I had to pick a castle essential, I certainly wouldn't go old school sets right now. As many have pointed out, the older torsos pale in comparison to the recent ones. If you are just starting a new castle collection, then stick to the recent sets that you feel fit your army. And yes, I would put MMV in the list of you just have to have as a castle builder. Just because as you build, you learn some techniques and it just makes your heart happy to build it. The Joust would probably run second just because of the elements you get with it are a good basic start for a castle collection. And if you can get a hold of a chess set that you like the figs from, then there is a really good place to start your army.
My other advice doesn't lend toward sets, but lots on ebay. Sometimes when others aren't looking, you can find lots of plants, animals or weapons, or hair pieces and they end up going for a nice reasonable price. Maybe they aren't 100% castle, but there are some good things to add to your castle builds.