At first, viewing the castle doesn't seem to offer you much space for building, but if you clear out the scenery and paths in the middle of the castle, you will give yourself plenty of space and a fair slice of money to start off with.
The first thing you should do is block off one of the paths leading to the entrances with a 'No Entry' sign and change the paths that form the bridge between the park entrance and the castle to being only 1 square wide. This prevents guests from going anywhere but into the castle. Also, delete the paths that lead to the castle walls as your guests will wander aimlessly around the castle and get lost easily if you don't.
Start your building by erecting all of the non-tracked rides you can - there only are 4 to start off with - and then build a small coaster and the Oak Barrel Ride. With a half-dozen rides in action, your park rating should stay above 700. When your park rating slips below 700 (and it will a couple of times in the first few months) don't panic, as you have 4 weeks to solve the problem, and either just keep building your rides to make guests happy or ensure they leave in a better mood by putting an entertainer next to the exit.
As you really only have roller coasters available to expand with, always make sure your park entrance fee is as high as possible as you need the cash. As you research new rides, put them in, but really only roller coasters will make the guests come. Don't bother building a few big mega-sized coasters as you don't have the space. Just build simple and compact ones and plenty of them. In the end, I had all 12 non-tracked rides, a Mini Golf, the Oak Barrel Ride and 8 roller coasters inside the castle walls which got me to the 1,500 mark at the end of Year 3. If you're struggling for guests, don't forget to advertise; it will help!