1. You should have verified about the unapproved tower before applying,
2. However, since the builder has accepted that there are unapproved towers in one of which they had reallocated a flat to you, you are in a comfortable position legally,
3. send the builder a legal notice to reallocate you the flat in an approved tower within 15 days,
4. If they fail, file a complaint case before the local District Consumer Dispute Redressal Forum alleging deficiency in service and unfair business practice claiming refund of the paid amount with interest, damage and cost.