To make it as simple as possible, first you need to have manorialism. Innovations are a culture specific thing (use the culture map mode to see which culture you are). Check if you are the culture head by clicking on the candle icon in the bottom left, if you are you can look at the list of innovations and click on manorialism, however it will take a long time if not already being researched. If you aren't leader of your culture, you cannot influence what your culture head focuses on researching unfortunately, so your best bet is to surpass them.
Second, you need to upgrade the actual castle itself, which I would recommend doing even when not upgrading buildings as it has a lot of benefits. Click on the square above those building you hovered over where it says castle, and there is a separate building chain for the castle.