A few people have asked whats going on at the Castle, things are changing almost weekly but here's the picture right now. Just to say that all restaurants and public WCs were open but the south gardens and imperial gardens were closed. Prague Picture Gallery was closed. ST Wenceslas Vineyard Street food at the top was open but the scenic sit-down restaurant in the vineyard was closed. I was at the castle today 1) because I wanted to take a look at the restrictions and 2) because currently it's free. Definitely consider two things.
Firstly you'll be stopped from going into any of the historic buildings including the exhibitions if you do not have a face covering for your nose and mouth. This does not need to be a mask, people with bandanas were getting in ok plus you'll have to disinfect your hands as well using the machine or bottle provided by the castle. Note that there are lots of places in town now selling basic cloth face coverings (elasticated) for around U$3.
Secondly is the queueing system. Each historic building is admitting a maximum number of people i.e. ST Vitus is 250, Old Royal Palace is 200. St George Basilica and Golden Lane is limited to 50. I visited the Old Royal Palace without waiting but at Golden Lane I stood there for 15 minutes and didn't move an inch. Definitely the smaller venues are going to have the biggest waiting times. So if anybody is booking tours of the castle with non-licenced Castle guides then any timescale they give you is completely up in the air because there's no way they can predict the queues and only the Castle guides can skip lines.
Hope that helps