"Adrian" wrote

The stations on the northern half of the circle may have made sense in
the 1860s. They are inconvenient today. And, said side of the Circle
misses interchange possibilities at almost every opportunity. The
biggest omission IMHO is not having a station in front of Euston. It
would be an expensive mistake to rectify.

IIRC the entrance to Euston Square Circle Line station is at the west end of
the platforms, and is near enough to Warren Street LUL station as to be
considered an interchange. As the Circle Line runs under the Euston Road,
and the buffer stops at Euston are a good way short of Euston Road, a
perfect interchange would be unduly expensive. However, a much better
interchange would be achieved if the entrance to Euston Square station was
moved to the east end of the platforms (or a subsidiary entrance provided
there). Incidentally, Euston Square station was originally named, more
appropriately, Gower Street. It was only renamed in 1909 as a late reaction
to the opening of tube stations at Euston on both the Hampstead Tube and the
City & South London Railway (now the Charing Cross and Bank branches of the
Northern Line).

Peter.