  We introduce the notion of city products of right-angled buildings that
produces a new right-angled building out of smaller ones.
  More precisely, if $M$ is a right-angled Coxeter diagram of rank $n$ and
$\Delta_1,\dots,\Delta_n$ are right-angled buildings, then we construct a new
right-angled building $\Delta :=
\mathrm{cityproduct}_M(\Delta_1,\dots,\Delta_n)$. We can recover the buildings
$\Delta_1,\dots,\Delta_n$ as residues of $\Delta$, but we can also construct a
skeletal building of type $M$ from $\Delta$ that captures the large-scale
geometry of $\Delta$.
  We then proceed to study universal groups for city products of right-angled
buildings, and we show that the universal group of $\Delta$ can be expressed in
terms of the universal groups for the buildings $\Delta_1,\dots,\Delta_n$ and
the structure of $M$. As an application, we show the existence of many examples
of pairs of different buildings of the same type that admit (topologically)
isomorphic universal groups, thereby vastly generalizing a recent example by
Lara Be{\ss}mann.
