HLN says despite Champions League qualification, Atalanta are prepared to sell the Belgian.
The Oribici accepts after two seasons Castagne feels ready for the next step in his career.
Napoli want to sign him as a back-up option and proposes an annual wage of €1.5m.
However, Palace are now in contact and can better the Azzurri's salary offer and offer the prospect of first-choice status under manager Roy Hodgson.
The Eagles would be willing to pay €20m to bring the defender to London this summer.