Install a second window for soundproofing

Well, I’d like to see the landlord who allows you to do that … I wouldn’t do it. Just the restoration and damage to the building are not even remotely covered by a deposit.

Besides, it’s pointless, the noise goes throughout the whole building. Like when the neighbor is drilling holes with a Hilti hammer …. there’s nothing you can do about it

Maybe it makes sense to think about soundproof windows right away?

Before you go into more detail, I would first test using foam sheets, a mattress, or similar to see if the sound entry through the window or heating niche would be noticeably dampened.

Given the expected structural effort, I see no chance of achieving anything with the landlord for your solution.

Perhaps special heavy and multilayer soundproof curtains could help? They are expensive but can be removed completely without any residue.

There are also proper curtains designed to absorb as much sound as possible. This way, you can also visually improve things a bit ;)

I don't know whether I should cry or laugh because of the initial question o_O :p