So I just hit 60 on my only character, a tower apprentice. Should I continue grinding hard mode Ramparts up to 70 or is there a better place to level?

It's good exp. I recommend just trying out insane maps/challenges that interest you. You will hit 70 faster then you think if your concern isn't 'grinding'. Try to join a good host, or host yourself. No need to be solo.

Hmm yeah I haven't tried any of these challenges yet; I'm gonna start doing those then, thanks a lot!

If i remember correct, for the future, 3 games of glitter on hard will get you damn near close to 60-70, if it is your towers that is.