In Hanamura there's a "ledge" above the right-hand side of Point B. Some characters can get up there themselves whether they're on attack or defense. If you have time between waves, Mei can use a wall to get defenders up there.
Based on my playing around, I've been able to get up there with the following characters:
- Genji (wall climb)
- Reaper (teleport)
- Hanzo (wall climb)
- Junkrat (landmine)
- Mei (build a wall under myself next to it)
- Widowmaker (grapple)
- Mercy (leaping to ally)
(I wasn't able to get up there with Lucio wall ride, but I suspect it's possible).
Any other characters or tricks that I've missed?