In reply to O'Donopoo:
The crux on castle ridge isn't really much easier than the crux of tower ridge to be honest (not sure if its avoidable from memory), sure less intimidating.
Not done ledge route in summer.
The best summer ridge on the Ben that I've done (North, Castle and Tower) that's just about a scrambling grade is tower ridge IMO. You could make it pretty safe with a waist tie in a few slings and biners and 30 metres of rope if you so desire.
If you get an early start castle ridge should be very doable before early afternoon for you last day. round trip of 4 hours walking max plus 3 hours leisurely scrambling. Leave out at 7am and you would have plenty of slack.
For reference it only took me 2.5 hours to do tower ridge from the CIC hut - we stopped to eat, definitely not rushing. Castle ridge would have been similar i guess.
Post edited at 17:24