Not sure how well I'd be able to communicate my knowledge on the subject but here is some general design theory on it.
In my experience when playing TF2 a good control point in KOTH is one that makes players want to stand on it.
Consider these examples one from Suijin and one from Harvest.
Suijin
Harvest
Both of these points are placed under severe height disadvantage and in the case of Suijin also feel quite claustrophobic. The problem here should be quite apparent very quickly Soldiers, Demomen or anything really can easily (and safely) kill anyone trying to capture the point. Whenever I'm playing on these maps I almost always just run right past the points, there is no reason I would want to be on them I can easily be killed, have little chance to defend myself and to top it off if I do capture it I'll just be giving myself longer respawn times.
People not wanting to go near the objective isn't something you want in any map. So let's see what Viaduct does instead.
Viaduct
The only height advantage onto the point is provided by the small rock in the middle, but it is easy for players to see anyone exploiting it and is no where near the height of Suijin or Harvest. Meaning anyone on the point will see the enemy (so they'll feel like they were fighting them instead of just getting killed by them) and because of the significant reduction in height they also have a chance of winning the fight.
Next the important thing Viaduct does is make the point area advantageous to be on by using a smooth height gradient to reach it. This does two main things for the map that are relevant in this context.
Firstly experienced players won't just label it as a death pit and as a result secondly there is a much higher chance of teams working together around the objective as opposed to just playing the game as though it was TDM.
The other thing you have to always be careful of is sightlines but that relates as much to other parts of a map as it does mid. Looking at the differences (which I'm not going to do because it is outside the scope of this post) between Viaduct and the competitive version of it (Product) is worth it when designing and understanding these. As a general rule you want around 2 sightlines onto the point with cover for players to use against the Snipers.
Also I looked at your map so I know you haven't done this but don't put health packs around the point otherwise the position provided by a good point can become too strong.
So in summery you want:
-Your point to provide a solid, advantageous position for players on it. Basically don't be Harvest, Suijin or Arctic. (Apologies to the mapmakers for the indirect attack that statement constitutes. You guys and your hard work is awesome.)
-To keep those gosh darn Snipers in check.
-To make sure that you have one (in some cases two) secondary way onto or around the point.
I could go into a lot more detail on stuff but I've babbled on incoherently long enough. Perhaps someone else will come along and say all this in a more clear manner.