I understand one's entry to the three major sites in the Alhambra is limited to before or after 1400, based upon one's reserved time to enter the Palace. My impression in reading Rick Steves' Guide is that parts of the complex have free entry regardless of the time on one's entry ticket and that there are just three turnstiles where tickets are checked. Is my impression correct? Thanks in advance.
It's a massive sight that has a few ticketed areas. You are free to roam around the site anytime, but cannot enter the ticketed areas except during either your morning or afternoon time slot. There are services available and a few other things you can do before or after your session. The Nasrid Palace itself is a time-specific entry. You MUST enter in the 1/2 hour window provided on your reservation or you will not be allowed entry (VERY strictly enforced).
Personally, I like the morning slot so I can have lunch afterwards, then roam around some before heading back into the city.
The Nasarid palace is strict regarding times.
Charles V palace isn't ticketed at all, so do that whenever you want. The Alhambra museum and the art museum are in there for a separate fee (a few euros), but no Alhambra tickets are required.
The Generalife is ticketed, but last visit I went in at around 14:30 or 15:00 when my allowed slot was that morning and didn't have trouble. I wouldn't 100% count on this, just my experience. I think Alcazaba is ticketed, but. I did that first so don't know how strict they are with the times.
Your impression is correct. The Nasarid palace is very strict. I remember the window being 15 minutes but will accept the other opinions that it is 30. While waiting a few minutes for our timed entrance I saw people being turned away who had missed the previous time slot. Very unhappy and some behaved as typical ugly Americans.