Castle Siege is a really odd counterpick that can benefit every character somehow, but somehow also put them at a disadvantage.
For Samus, clearly, that is the second part of the stage. But, I'll get to that in a second.
Stage Part 1:
This is actually not too bad. You have the slope, so keep out for that when using your projectiles, it screwed me over some time already.
The platforms are nice for Screw Attacks or UAirs, and can prove very useful. It's basically a little bit adapted Battlefield. It has a high ceiling, but the side-blastzones are a bit closer, so it might be useful. Getting a KO here can be tricky, if you kill someone here' it probably going to be on the second run-through, unless you're lucky. Not too bad, but not insanely great either.
Stage Transition:
This thing is probably one of the most underrated parts of that stage ever, lol. The transition can be used for quick KOs with ZAir chains or Charge Shot. Be careful, though, as those are walkoffs.
Stage Part 2:
Now... this is horrible for Samus. First of all, this thing is HUGE. The statues block your projectile game by a big margin, so you should stay in the upper half of the stage. Be careful, as there's a whole bunch of walkoffs, so don't let a Dedede grab you here.
If you are good at staying and defending a position, I'd suggest staying in the center of the stage, on top of a statue. This is probably the worst section for Samus. The ceiling is even higher than in the first part, although this section has the closest side-blastzones on this stage.
Stage Part 3:
Now, this section is one single platform that's tilting. Watch out, as it's not completely even - there's a small step that lets the left side of it be a little higher. Just something to keep in mind - not that it's a big problem.
I forgot how high the ceiling is on this one, I think it was the lowest in the stage; but the side blastzones are the farthest away. This section is great for camping and ZAiring. Otherwise, the tilting can screw a bit with your recovery, so be careful.
In general, this is probably the most uneventful part of the stage.