This is my first attempt at a bigger model, and I'm seeing that some areas seem sunken in while others look bloated, and it makes the towers look off. Is there a way to even out the ground so parts don't look twisted?
I'll just add what I was taught, when building, if the area will be full of air, try adding just enough tissue paper to take up the space. to prevent bowing out, think about adding formers inside. @Rhaven Blaack , and @Revell-Fan can offer more suggestions. to me this castle looks pretty good.
To prevent sagging I use a lot of leftover card from cereal boxes, corrugated card from shopping boxes and so on. Recycle what you can but first make sure that the card you use does not smell.
You can reinforce straight areas by directly laminating pieces of card to the back side. It does not need to be pretty because you will never look at it. If the kit does not come with formers you'll have to make them yourself. This is a case of trial and error. Any shape will do as long it is able to give some stability as you can see here.
I can see that some of the walls are warped. This can have several reasons. If you use water-based glue it is best to use it sparingly or the moisture of the glue will make the card / paper too wet and flexible. If the card is too rigid for folding you may end up with creases. Pre-shape curved parts before you laminate them. Always take your time during assembly. Building is no race but should be fun.
A lot of the sagging comes from the need of stiff objects, like skewering sticks stuck in key spot. The big problem comes from moisture though. If you seal the model, I would say use a 'Flat Clear Coat' paint, with a couple of layers, and do the whole model, edges, bottom, inside, outside, pieces that can be done while or before gluing. Then the moisture will not get in and the model will get stiffer. Some areas can be stiffened with "Crazy Glue', though, due to the noxious fumes, I don't use it.