Tom Holder
Google
Entry to the museum is €10 on a combined ticket with the nearby Ggantija Archeological park (the latter makes is worthwhile) or €13 as part of a combined ticket that includes 5 Citadella museums in Victoria (Citadella visitor centre), museum of archaeology, old prison, Gran Castillo historic house, and nature museum) as well as the Ggantija Archeological park and Ta Kola windmill.
The windmill makes for an enjoyable 15 minute visit, but I wouldn't trek out to Ggantija just to see it.
It is also wholly unsuitable for people who cannot handle lots of twisting stairs (you can see it from the outside though).
The windmill is prettier on the outside than the inside, so you may not wish to actually pay for entry of you don't have a ticket for the nearby Archeological park.