Halcyon Castle — now called Kovalam Palace — is the most architecturally distinctive building in Kovalam, sitting on a promontory slightly north of Samudra Beach with its towers and crenellations visible from the water. Built in 1932 during the Travancore princely state, it was designed as a royal summer residence and was one of the first grand buildings on this stretch of coast. The design is a peculiar Indo-colonial hybrid: whitewashed walls, tower rooms with narrow windows, and a wide veranda that faces the sea.
The building has operated as a hotel at various points in its history and is worth knowing about even if you are not staying there. The grounds are accessible, and the views from the promontory on which it sits are among the best in Kovalam — the headland gives a clear sight-line south toward Lighthouse Beach and north toward Samudra. The Kovalam Wikipedia article identifies this as the notable historic building of the area, and it is one of the few pre-tourism-era structures left standing on a coast that has otherwise been rebuilt entirely.
Access depends on current usage — it has changed hands and function multiple times. Check locally whether the grounds or the building itself are open.