As Peach, you can never really go wrong with Castle Siege. She has enough playstyles to handle ANYBODY on this stage. Her ability to avoid grabs via float makes the walk off not a huge deal against chaingrabbers like Falco and DDD. This is my counterpick of choice against MK and several other characters.
I like how huge this stage is. What ends up happening is everybody survives a bunch; including Peach, minimizing her inability to kill. In T2 (Transformation 2) she can peach bomb the pillars as well as refresh her moves. In T1, the platforms are basically designed for her up tilt and forward smash combos. T3 is great for bonewalking. I really love this stage. I'm confident against anybody here.
I enjoy playing Marth here because of how big of a stage it is. Marth likes BF and Lylat - smaller stages with tons of platforms. 2/3 of the T's on this stage are HUGE, and the other one is slanted permanently, it's not exactly ideal for Marth. You can plenty of space to run away and play your turnip game on even the first T, unlike BF and Lylat. It's not a counterpick against Marth, it's just better than Smashville. It all depends how you choose to use the stage. marth doesn't have the variety of playstyle Peach has so I think she is the one who has the advantage as long as she adapts the right playstyle. (For example, a Peach can play completely long ranged with turnips and bonewalking, and stay away defensively. A marth eventually has to approach, so he can't quite mix it up as much) This mix up game is excellent for adapting to each transformation as Peach.
You won't find it a good stage if you choose to play the same playstyle on each transformation. The key is to switch it up. Coupled that with the survival/kill thing I mentioned earlier, the small interferences that can help, and the large ceiling, and this is a solid Peach counterpick and IMO the best against MK.
Reasons its good against MK:
-T2 and T3 you will never be gimped by shuttle loop. Anytime you are in danger of being edgeguarded just recover high, MK's jumps are too slow and he won't be able to chase you like he does on Smashville or BF to the top. Plus the ceilling is huge. I got hit by shuttle loop while recovering towrads the top, and DI'd up, at like 160% and survived before. MK player got pissed. It removes Peach's biggest weakness on MK and that is getting back on the stage.
-T2 doesn't even have edgeguarding. MK without an edgeguarding advantage = not so great. Also you can fair kill MK here rather easily since they are walk offs. True he can do the same to you, but now you are even in kill aspect, rather than him having the advantage.
-Camp like a MOTHER on T3. Turnip abuse on MK. There's just so much space, even tornado isnt' that effective, because you have too much space to operate and deal with it.
-T1 makes it hard for MK to approach you becuase of up tilt and f smash, if you hide under the platform. Try to stay on the right side, its favorable for characters with projectiles as they will fall down on the other opponent if thrown forward. Also MK cannot abuse his recovery too much since the stage vertically does not go down too far compared to neutrals like FD.