In reply to jrclimber:
Some good advice here. From the perspective of a local who has done all of those ridges quite a bit (for work) and seen other folks on them:
They are all long and the weather affects them quite a lot.
Castle is the most exposed to bad weather from the west and although its a Mod many people find the cruxes at least as nippy as Tower Ridge.
Tower Ridge is long and if you end up off route what was quite clean rock can become a lot looser, more vegetated and slippery in a hurry- especially in the wet. Understanding of what you are happy soloing and good mountaineering rope work; the ability to change from soloing to pitching or even moving together will all help a lot. (although this only adds speed if the party is good at placing and removing gear quickly and done badly can be frighteningly hazardous).
NEB and Observatory are a step up in seriousness. more loose rock, greasier in the wet than Tower and with harder and more exposed pitches. A good test of mountaineering efficiency for most and especially in anything less than ideal conditions.
The CMD is a nice long easy scramble. Ledge Route can be done by the harder route (some loose rock and a short slabby section with mediocre holds) or an easier route (long pleasant Grade I) and would be good for a solo outing to get a feel for the mountain and the terrain.
Warming up on something a little shorter like Curved Ridge on Buachaille Etive Mor might also be a good idea to 'test the waters' on mountaineering routes looking at route finding, route reading with regards to what you are happy soloing and deploying the rope efficiently if needed.
Feel free to PM if you want more advice (although I guide the routes for a living I'm not above dissing out some free beta or conditions information).
Al