Yep that'll fit, King David.
The baseplate will stick out a bit front or back (or a bit on both), but I'm sure you can add some grass (normal green plates) and plants to make that look nice. But the baseplate will make a nice base for you to add tiles and interior walls or a well or some fences or anything for that matter.
Just keep in mind that that big baseplate is quite bendy and won't make for a stable base if you plan to pick the castle up after attaching it to the plate.
What I usually do with a creation on a baseplate is move a little wooden plate under it to be able to lift it up without bending the baseplate and making some walls tear off as an effect of that.
Between plotting to kill you all and chasing balls of yarn, I also build MOCs