Nothing is square, and that is fine
Out-of-plumb walls make rigid enclosures and large-format tiles difficult, because both expose deviation. Adjustable-profile enclosures, smaller tile formats, and a scribed rather than butted approach at junctions produce a far better result than trying to force modern components into an old room.
Floors and load
Suspended timber floors in older homes often deflect more than modern tiling systems tolerate. Strengthening or overboarding is normally required before tiling, and a cast-iron or stone-resin bath adds significant point load that should be checked rather than assumed.
Moisture and breathability
Solid stone or lime-plastered walls behave differently from cavity walls: they need to breathe. Sealing them with impermeable finishes across large areas can push moisture elsewhere. Where a wet area meets a solid wall, the detailing needs thought - and in listed buildings, formal consent for alterations may be required before work begins.
Pipework you inherit
Expect a mix of ages and materials, sometimes including lead or steel sections. Routes were often chased and re-chased. Allowing time to rationalise pipework properly during first fix costs less than leaving a compromise buried behind new tiles.