A handful of things might do it -
- a score of mages powerful enough to cast anti-magic field with their martial bodyguards astride pegasi/hippogriffs/gryphons might be enough to get a handful of fields inside the tower and disrupt the inner workings.
- that same score of mages might participate in a mass scry/buff/teleport routine to the base of the tower, bringing with them clerics, fighters, and individuals capable of breaching the wall of the tower itself to grant access. Judicious use (and maybe multiple layers) of Wall of Force could protect the group long enough to get inside and cause havoc with anti magic, dispels, and good old fashioned butt-kicking (and, once inside, if a friendly Arcane Archer happened to have a quiver full of arrows and the will to use them to dispel magic from afar could come in quite handy...).
- a score of mages powerful enough to cast anti-magic field with their martial bodyguards astride pegasi/hippogriffs/gryphons might be enough to get a handful of fields inside the tower and disrupt the inner workings.
- that same score of mages might participate in a mass scry/buff/teleport routine to the base of the tower, bringing with them clerics, fighters, and individuals capable of breaching the wall of the tower itself to grant access. Judicious use (and maybe multiple layers) of Wall of Force could protect the group long enough to get inside and cause havoc with anti magic, dispels, and good old fashioned butt-kicking (and, once inside, if a friendly Arcane Archer happened to have a quiver full of arrows and the will to use them to dispel magic from afar could come in quite handy...).