History
A site at Mont Cassel was used in the early 1950s for a temporary cross-channel televsion link to Dover. Research into this link is ongoing.
The "Cassel" tower (located at Saint-Sylvestre-Cappel) was constructed for the 1959 permanent link from London via Tolsford Hill. This developed into a high-capacity telephony route, with connections to Paris and Belgium via Lille.
The type of structure with circular "galleries" around a cylindrical core and a separate building contrasts with earlier French sites which had the equipment in a square tower with the antennas mounted on platforms. An article in Electronique Applications No 5 (Spring 1978) comments:
"Between 1956 and 1962, [...] two pylons were erected on the Lille-Great Britain line: Fiennes (30 m) and Cassel (41 m)." Possibly these were the first to use this design.