Big ol' feedback dump incoming.
First thing's first: you've got a leak. A leak means that there's a way from inside your map to the outside, or that the inside of your map isn't "airtight." Imagine filling your map with water - if water can spill out, that's an issue.
Why is leaking an issue? Because of how the Source Engine works, having a leak means the engine can't decide what the inside or outside of a map is, and in turn can't do "visleaf optimization." Visleafs are a kinda complex topic, but the short version is - the entire inside of your map is divided into a bunch of shapes, and the engine can decide what needs to be drawn and what doesn't by using them. If it can't make these shapes, it basically means everything renders all the time. Also, the lighting engine will only do a very rough pass over things, and light can't get "bounced around" like it should. That's why
this is happening. You'll also notice that
sometimes the skybox will start doing funky things. This is also another sign of a leak.
How do you find and fix leaks? In Hammer, find Map > Load Pointfile. Assuming your map is leaky, it will draw a red line showing where the issue is.
Now, on the actual layout. It's not awful! However, your scaling is
huge. Getting the scale of a map right is very important, but also pretty difficult. Some people find it easiest to look at official (or good custom) maps for a sense of what scale works well.
For instance, in your map, the mid building is pretty big. Getting from one end to the other is going to be a bit of a challenge if the other team has a sniper, or any class with decent range really. I would look at Sawmill's mid building for an idea of what a good-sized mid building would look like.
Another thing about your layout is the lack of height variation. On a micro scale, you've got a good number of ways to change height levels and get some interesting gameplay going. However, there's not a lot of macro-scale height differences. The height level from one spawn to the next doesn't change anywhere between. If you look at Viaduct and Sawmill, the ground generally slopes upwards towards the point. Other maps have height advantages within an area rather than as part of a route,
like this area in Upward - it helps to give players more options on how to play in that part of the map.
I'll be honest, this map is pretty crappy as it is now. However, what you've got right now is an excellent learning opportunity. You'll probably get some nasty feedback from people, but those people are jerks.