Based on someone who used to dominate castle, retired.
1. Getting someone to kill you in halls might take a while sometimes, so you are better off just dying in flag room, since horses don't lose their bush speed.
2. Bombs drop when you die. I'm not rich, so yeah.
3. I have seen people shoot arrows behind stair blockers, and it's pretty easy to just evade them.
4. Statistically speaking you get more pks at MoD consistently over Castle, since the time to get to flag is shorter, and it's more crowded. Sure you might get a lot of pks in castle over time in one life, after you died a bunch and have a lot of people in flag at .5-1.
5. I think it is more fun to pk at castle though, since it is less chaos. I used to run away and not let people kill me, and they would get pissed. Then I would go heal with the pots, before ap was a thing on iclassic.