How do I 3D trim this?

[Zox]

So my case is this

 

 

I want blue object to stop protruding after red arches. I would usually make copy of red object somewhere on the side then use "solid subtract" to cut blue object the way I want it and the blue stuff floating above arches would get rid of by "separate" command but this doesn't work in this case.

 

Can someone give me some tips on what to do?

[ReMark]

I would take one section of wall and slice it using a profile of the arch using the object or surface option. Then I would array the wall section to complete the other sides.

Edited September 21, 2017 by ReMark

[Zox]

ok here you go

3d review.dwg

[ReMark]

Sorry, had to go with my own version.

 

[Zox]

I guess I don't know how to use "slice" option: I choose "slice" then I chose the blue object and then I choose object or surface but I don't know how to choose profile of the arch. I click and click and nothing happens.

[ReMark]

I used the profile of the arch BEFORE it was extruded. And I sliced only one section of the octagonal wall.