Castle Siege is definitely a good stage for Shulk.
To begin, here are some sources about blast zones:
http://smashboards.com/threads/wii-u-stage-blast-zone-data.395357/
http://smashboards.com/threads/ceiling-height-research-thread.384187/
Transformation 1:
- Easier kills for us
Shulk is primarily a horizontal killer - F-smash, F-tilt, off-stage Fairs, Bair, Air Slash, D-smash, D-throw/B-throw. Of course, he does have some capability for vertical kills: Uair, U-smash, U-tilt (more Monado Smash though), and Mighty Air Slash (possibly regular air slash if high enough). Those kills are rarer, so Shulk relatively benefits more from horizontal blast zones being thinner. This transformation of Castle Siege has the thinnest blast zones from the center other than transformation two, and is still pretty thin when compared to the edge of the stage as well (185 makes it only beat by Halberd stage two and walk-offs, for legal stages).
- Improved (or hindered) spacing.
The slant on stage in the middle can be used to improve spacing if you are attacking an opponent who is on the right side of the stage. When you land on it, you will slide away during your landing lag, making you hard to punish. A reversed Bair is threatening while in that position.
- That slant can help our Nair combos to connect.
- When on the lower part of the slant, the second slash of Advancing Air Slash will hit grounded opponents.
- Try to remain on the bottom portion of the stage.
There are just more options when you are down there.
Transportation part:
-Flat stage + Walk-off
Try to exploit this for the short time it lasts for an easy KO. A Monado Speed pivot grab might just do the trick. Lasts only 2 seconds.
Transformation 2:
-Statues = Win against projectiles
You can attack and maneuver through the statues, but most projectiles are stopped by them This makes them great cover against projectiles.
-Walk-off
The walk-off is nice for monado speed pivot grab B-throws. Also, D-throw -> Dash -> Fair can land some fantastic kills.
-High, plentiful platforms
Useful for stalling in Monado Jump (great for waiting for Shield art's cooldown to finish).
-High ceiling, thin horizontal blast zones
This stage actually has the thinnest blast zones from its center, AND it's a walk-off. This stage has the earliest horizontal kills of any. Its vertical blast zone is only beaten by Kongo Jungle.
-Notes:
Be aware that the statues can screw up MALLC timing. They give extra hitstun.
Buster deals more damage to them. Be cautious about that. They have about 60 HP.
The cloth platforms at the top of the stage go off-camera, but are not walk-offs.
Transportation part:
Same deal as the first transportation part, but if too far towards the sides you could end up off-stage when you arrive at the third transformation.
Transformation 3:
-Horizontal kills still easier, but not as much.
Horizontal blast zones are a little thin from the center, but not as much so. However, they are just about as thin as stage one when compared to the edge of the stage. Relatively average vertical blast zone.
-Tilting of stage can help or hinder spacing, and help or hurt Nair combos connecting.
Yep, this again.
-Tilting of the stage can mess up projectiles
-There's a slanted bump on the left side of the stage.
-No platforms
Transportation part:
Same circumstance as the second transportation, but try to be even farther from the edges, since the first transformation is smaller than the third.
Other information:
-Each stage lasts about 43 seconds, so approximately 2 decisive arts.
-The transportation part can save you from dying on the bottom blast zone.
-Stage shakes for 3 seconds before transforming. Use this to time your BA off-stage back slashes XD.
-Transportation part lasts 2 seconds.