Certain conformal field theories support multiple levels of useful
(semi-)holographic descriptions which geometrize different sectors of
the CFT, and this `double holography’ provides a vantage point e.g. for
black hole studies. The CFTs involve lower-dimensional degrees of
freedom localized on boundaries, defects or interfaces within a
higher-dimensional ambient theory, in such a way that the defect degrees
of freedom are holographic both in their own right and in combination
with the higher-dimensional ambient theory. In this talk I will review
the realization of double holography in Type IIB string theory based on
4d N=4 SYM with Gaiotto-Witten boundary conditions, and discuss recent
work exploring the field theories and their interplay with holography.