Noun adjuncts, possessive or using "of"
I'm intuiting that it's to do with compositionality - that is, the Channel Tunnel entrance is a component of the Channel Tunnel, but the top of the Eiffel Tower isn't a part of the Tower - it's a location on it. Just as you can't say the Channel Tunnel middle/end/beginning. Channel Tunnel start is questionable.
I think the explanation would be:
- If the entity is a component of the whole (Eiffel tower base/stairs/lifts) then you can use it as a qualifier, otherwise;
- You need to use it as a focus (the bottom/top/middle/outside of the Eiffel Tower)