I am of the opinion that the A backward hold could be made much more fun with the addition of a simple wall and doorway here.
You tend to find that RED close-range classes congregate on the high ground. Therefore, you can extrapolate that RED will find it more fun if the BLU sniper perch cannot see that high ground. That way, close-range classes are fighting close-range classes, and long-range classes are picking off whatever RED stragglers decided to use the long-range, low ground route - usually other long-range classes, as it turns out.
This particular BLU sniper perch can also snipe RED on their way to the high ground from B, which is not fair or fun.
To fix this, you could add another wall+doorway to block the orange arrow, but there's another solution which I prefer quite a bit:
This would allow RED to guarantee entry to the high ground without passing that sniper sightline, and it would also give extra meaning to these fences, which previously existed to block a sightline from the B lobby, but were pointless since you would have to walk into the door the sightline saw through anyway.
This solution also lets you upgrade the B lobby:
If RED tries holding the old doorway, they're letting BLU get behind them via the new one. If they try holding the new one, they're letting BLU get behind them via the old one.
This forces RED to fall back and hold at the only place which BOTH doorways go into, which happens to be the square I will later mark as the B forward hold. This makes the B lobby impossible for RED to hold - a desirable effect for us.
However, since I've already argued that you should add more space between A and B, this solution couldn't be implemented exactly as I've drawn it. It's just food for thought.
Currently, RED's routing from their first spawn to the B backward hold is not cool, and gives them a massive disadvantage.
You get two doors, but they lead to the same room. Furthermore, that room only has one quick door to get to the control point.
The massive rotate time between the two spawn doors compared to the nonexistent rotate time for a demoman standing outside your spawn to watch both doors, as well as the single-door choke, makes RED's life hell as they try to respawn and enter the backward hold.
This is admittedly not a high-priority fix, since if RED has been pushed back this far, it logically follows that they've already lost the backward hold fair and square and should subsequently lose the point. It's just, again, food for thought, and I hope it is indeed nourishing.
The C lobby suffers from kind of a combination of the past two problems.
There are two distinct entrances, and they're reasonably far apart for BLU. But for a RED player occupying this lobby area, they're not far apart at all. A true lobby area should really be split into two halves like I described earlier, so it's completely impossible for RED to hold one route without risking a backcap from the other, so they're forced to fall back to the forward hold, where the two routes meet.
C has a minor case of sightline blocking that's maybe not as effective as it could be.