Ynysmaengwyn was a fine brick house rebuilt for the Corbett family from 1758, with some earlier 18th-century agricultural buildings being retained, the whole forming an exceptionally fine 'U'-plan group. Two-storey, main block flanked by quadrant shaped walls each of five arched bays, joined side wings facing each other. Many pure eighteenth century features, rare in Wales.
The house, which had been left to the local authority, was neglected and demolished in 1964. The ballroom wing was demolished as late as 1989. For associated structures at Ynysymaengwyn, see NPRNs 41757, 54224, 54225, 28894, 28895 and 265175.

Tywyn 4 (NPRN 419550) is currently located inside the west end of St Cadfan's Church (NPRN 43861). It was first noted in 1986 after the demolition of Ynysmaengwyn mansion (NPRN 54223), where it had been built into one of the outhouses. It was originally a sundial and is thought to have been associated with St Cadfan's Church. Its primary inscription is thought to be eighth to ninth century in date. In the eighteenth century it was used as a milestoneand is thought to have probably stood on the A493 where it passes Ynysmaengwyn.

Sources include:
Cadw, Listed Buildings Database

J. Archer, RCAHMW, 31.12.2004