Ok here's the feedback.
The first area was just impossible to defend with a sentry, try at least to create some spot where you can put a sentry, the building at the right can be a good spot.
Need a door entrance here.
To here.
Same goes here, need a sentry spot.
Try to prevent the RED team to go here, and make a stair at the other side for the BLU team to acces it.
This area need something more, why not trying a small catwalk under the bridge, like upward.
Make a sentry spot here and a path to acces it.
This house shouldn't be a chokepoint, make it easy to go trough and add another path.
Make it easier
This part was bad, you should scrap it and make something more fun to defend, I give you some ideas.
Overall the layout isn't bad, but the pacing isn't well set, Payload works like that:
-> easy to push -> chokepoint with sentries to block the cart and where the red team as an advantage -> easy to push -> chokepoint ....
Each of these stages need to be well determinated, when you play badwater by example, you know where the cart will block and where the red team will be, you even know where the engineers will put their sentries, that's a chokepoint. When you've been trough it and then capped the point, there's an area where the BLU team will push easily and nothing happen, during this time the RED team can create their defences at the next chokepoint.
Your map lacks of this, the cart is going trough the map without really stopping at a part wich is determinated, that's not fun.
If you want to create these chokepoints, just do the stuff I draw, and put some more ammo near the sentry spot, your map lacks of it. Don't be scared to give a lot of ammos to the players, more is better in this case, same for the health.
About the last point I feel that you need to scrap it and create something new, it was obviously overscaled, and not fun at all, create a cool geometry were the BLU team can attack from all sides and all highs, on the other side, create a lot of sentry spots for the RED team.
About your texturing change some of the textures, because it was really boring, and you don't really know where you need to look at, I use orange textures for the thing that the player need to look at.
Overall the layout isn't bad, and if you can fix the flow of the players, and the chokepoints, and btw adding some variety to your geometry (more highs/angles), it can be a pretty cool map.