I have read reviews that the lines for the alcazar in Seville are chaotic and long. I have booked tickets for entry into the royal apartments at 10:30 am. The website says entry allowed into the complex starting an hour before. I noticed for other tickets, entry allowed starting at 15 minutes before your time slot. How do they keep things straight? Has anyone been in last year that can provide guidance? I have read about people missing their entry because of being in the wrong line. It sounds stressful.

Ime experience the lines can be very long if one arrives at 11 or afterwards.
I always have purchased my tickets online just to avoid any wait.
If you purchase tickets for the 10:30 entrance, yes, you can enter the complex an hour prior. But when I visited the Royal Apartments, I purchased my tickets for 10:00 and went directly to the apartments ahead of time, going through the line at the Puerta del León, then up to the second floor to the entrance to the Royal Apartments (not well signposted so I had to ask). One has to arrive at the Royal Apartments 15 minutes prior to the slot for the obligatory 50-minute escorted tour as one needs to secure the English audio guide and you must stash your camera, phone, bags in the lockers provided. For that you'll need one euro, that will be returned to you.

I'm looking now at the official website, and there is already very limited availability for those Royal Apartment visits in the month of October. They do sell out.

Thanks, Maribel. I purchased my ticket weeks ago. Was there one big line for ticket holders when you went? I read reviews that said there were lines for different times. I want to be sure to be at the royal apartments extra early since I have heard of people being turned away even when 15 minutes early. Did you feel the locker situation was secure? Do you have to go through metal detectors to get into the apartments after checking bags? I’d rather have my cell phone on me. I would certainly not take it out during the visit.

Yes, you do have to go through a security control.
About the lines, it's confusing so I asked which line to stand in. Now the Patio de Banderas line is only for purchased for that day.

Because we purchased the very first time slot, at 10 am, we just stood in the line for the first visitors.

Once inside the Alcázar we didn't stand in another line to pick up an audio guide for the downstairs visit, but instead we went directly to the staircase that leads to the upstairs Royal Apartments. There we needed to check our phones (could have kept them in our pockets but didn't), bags, backpacks in the lockers and knew that we needed a euro coin (which is returned at the end). The lockers are perfectly secure. Security is tight there because it is still considered the Royal residence when the King and Queen are visiting Sevilla. The group is escorted inside by a security guard, and he/she indicates when to move from one room to another.
We also needed to take our passports, and they checked the tickets which had our names on them, as you know since you already purchased your tickets.

About your arrival time upstairs, if you've read anecdotes about ticket holders being turned away even when arriving 15 minutes early, that wasn't our case, but we did arrive in plenty of time, about 20 minutes before our guided visit.

This may be the post from the Rick Steves Spain forum that you´re referring to ??

"We recently toured the Royal Apartments. You really need to be there 10-15 before your tour time. We got there a few minutes before our time and they were waiting on us.
We were only allowed to take our phones on tour and they had to be in our pockets. Everything else had to go into the locker. You are not allowed to take any pictures. We really liked the tour."

Thank you, Maribel, for the additional information.