Let's see that definition...:
Allegory: The representation of abstract ideas or principles by characters, figures, or events in narrative, dramatic, or pictorial form.
Surely much in the Land is representative of some abstract idea or other; isn't the Land representative of health, the Illearth Stone of corrupting sickness?
An author does not have to purposely create allegory for his works to be justifiably read as such. Perhaps his thoughts comprise such a complex web of meanings that us readers can only do our best to understand him by proposing allegories of sorts.
But let's get back to TCTC. I think that beyond the allegorical aspects of this work there is an even more complex theme here: the entire story - as mentioned before by others - talks about allegory itself. TC sees the Land as an allegory for his own psyche. SRD does his best to keep us guessing as well on this matter. I think that the final verdict is that it doesn't matter if the Land is real or 'just in his head'; what matters is how he feels towards it, what it means to him and how it affects him. Taking that to the issue of allegory for us (and not TC) - it doesn't matter in the end if we call TCTC an allegorical work, or not; what matters is how it affects us. It certainly affected me.
