I had the exact same issue. What worked for me was starting another mission. About 30 seconds into the new mission I got a message saying to come back to the castle to finish up. When I got there Preston was there waiting for me. Hope this helps.
There's a related mission with the military guys, can't remember their name, where they also need the radio tower to be powered on. If you go and join the military guys you will eventually get this mission. This should allow the castle mission to resume.
Hold one, I'll get the mission name for you if I can find it
Can't find the exact mission, but the other group you need to join is the Brotherhood of Steel. I've done 2 missions for them. 1 of the missions allowed me to complete "Taking Independence". Good luck and let me know if this works for you.
Cheers
I had problems with this mission too. So bad I ended up loaded a previous save and losing a couple of hours play.
My problem was that after killing the queen it told me to talk to preston but I couldn't because he had somehow got himself trapped inside the wall.