A good start!
More health/ammo kits, even if they are small ones, that are on ground level. Height is an advantage in itself, give those down below something to hold down their for.
Give small ammo kits to engi setup placements, because engies are almost vital on ctf maps, and metal is vital to an engineer.
Your straight paths are far too straight and boring! The pipey vents are boring, and easy to spam. Add some turns, ups and downs, or even cover. Anything!
Remove access from the main inside area (outside of spawn) to the middle of the vents. It defeats the point of going round a little bit, or consider those exits onto the middle section to be closer to their relative bases, rather than directly in the centre. (The middle pipe would require being slanted then though).
Consider giving access to the upper areas at middle directly from the middle point, so that pyros and heavies dont get outclassed by snipers every time. Not every heavy wants to go round massively every life.
The lower area is extremely risky, being a non-straight linear path from base to base, with the added danger of enemies above at the centre. Maybe consider removing the connection between these tunnels; simply allow teams to jump down into them from middle, like cp_freight has with the water areas. Your safe side drop down takes you to the enemy base, and doesnt connect with the opposite sides tunnels!
Dont allow players to stand on fences.
Dont place flat signs on curved surfaces (the intelligence sign on the spire ironically
)
There are some strange shadows outside near the trucks and such. They disappear on my PC settings, depending on the angle I look at them. Very odd.
To reitirate, you HAVE to make the tunnels wider/ easier to navigate safely. Itll be a frustrating nightmare to use them in their current state.
Just some suggestions, dont feel the need to change anything at all, its merely my opinion.