Complex sentence: The Cochin Raja continued to send his tribute to Tipu as usual, but in the meantime secretly assisted the Kingdom of Travancore to build and strengthen their Northern defence lines (known locally as Nedunkotta Fort), partly through Cochin territory.
Split into simpler sentences: The Cochin Raja continued to send his tribute to Tipu as usual. Simultaneously, he secretly assisted the Kingdom of Travancore to build and strengthen their Northern defence lines (known locally as Nedunkotta Fort), which passed partly through the Cochin territory.