Is Thiruchendur the only lowland abode of Lord Murugan? I thought Lord Murugan's temples are always on a highland like in hillocks, hills, etc.

 1.If you closely observe, you can that Tiruchendur is in a mountainous tract.

2. It is a Karma Bhoomi. “The Victory of Lord Murugan over Surapadman”. Or Some cosmic event, Therikkad formation, represented by the war.

3. It is believed that the demigods (read devotees/ sanyasis) of Muruga wanted to worship him in a place where there was a mountainous tract, sea and river and hence Tiruchendur was chosen.