>>If the field is defined as nvachar(...) you don't really need to add N' in front as the field still will be nvarchar. You need N' if you use unicode characters.
>>
>>sp_ExecuteSQL requires nvarchar parameters (Command to execute and Parameters Definition)
>
>The declaration is:
>
>

>DECLARE @Command nvarchar(4000)
>

>
>So, at the end, if I have it or not, because of that declaration, it will not bother anything. But, as a best practice, I guess it would be better to keep it there.

You don't have to have N'ALTER TABLE

and 'ALTER TABLE ...
will work exactly the same as the command is declared nvarchar. So, you may safely drop it in this particular case if you're only using 'English' letters.