I have always wondered how the National Assembly got into the tennis court to make the Tennis Court Oath. I have asked professors and Googled this question and haven't found a sufficient answer. I just wonder if there are any accounts from members of the National Assembly on how they got in, or if there is any other information on this. Thanks!
It seems like they just walked there. I don't know if the building was normally left unlocked, but the deputies don't seem to have had any trouble worth noting getting inside. Since a suggestion to break into the locked hall was rejected (source), I'm assuming that the tennis court was open and chosen because it was easy to access.
According to Simon Schama's Citizens and Ruth Scurr's Fatal Purity, the tennis court was owned by a friend of Dr Guillotin, who made the original suggestion.