Castle Siege seems to combine a bunch of elements from other neutral stages, as well as adding some new ones. I'll divide this up for each section:
1st part - Similar to Battlefield in concept. Two floating platforms on either side of the stage allow more aerial-based characters to camp, avoiding your grabbing game altogether. However, the lack of a higher third platform means that you may be able to do a SH-aerial to disrupt them somewhat. The middle of the stage also has a slant in it, but I don't think it disturbs the ICs' grab that much (it may make D-throw chains harder when going down it, but that's all I can think of).
Transition parts - I'm putting all three of the stage transitions here for convenience. These shifting parts add something not seen in neutral stages: walk-off edges. Can you say "chaingrab through the blastline for an easy KO"? Depending on which stage you're switching to, there are two ways to perform here. If going to the second part, which also has walk-off edges, you can start an alt. F-throw chain during the transition and continue it until the target is KOed. If going to the third or first parts, which don't have walk-off edges, you can start a CG in the transition, forcing the enemy as close to the blastline as possible. Once the new stage is about to appear, knock the opponent away and hustle back to the platform. If the enemy isn't killed outright, you can set up an edgehog/guard once you recover.
2nd part - As described above, walk-off edges are great for CGs. As far as I can tell, the statues don't disrupt throws (correct me if I'm wrong, of course), so your grab game should be safe on the lowest level. However, this room also has many platforms, some of them high up, so aerial-based characters can avoid you with little to no effort.
3rd part - Back to the single floating platform idea here. This stage is about the size of Battlefield, leaving you some decent room for CGs. What makes this even better is that there are no extra platforms to escape to, making it a lot like FD (one of the ICs' best levels). The entire stage tilts somewhat, and there is a small bump on the left side, but these don't affect your grabbing game much either, just like the slant on the first section.
Overall, this is a pretty good one for the ICs. All three sections give enough room to make CGs practical, and the possibility of walking-CGs through blastlines can set up easy KOs. Extra platforms on the first and second portions of Castle Siege mean that enemies can more easily avoid being grabbed, although there are ways around that on the first section. On a side note, the transition parts may end up "saving" you if they appear right after you are knocked offstage (it will break your fall) - keep in mind that this can happen to your opponent too. Because the stages change enough to warrant slightly different strategies on each, long-term projectile camping is not very feasible, but desynching is still good for forcing the enemy to approach, setting up a possible shield-grab.