Hey there, and thanks for checking out my adventure!
I think this adventure would be perfectly suited for first-level characters, provided you have five or six in your party. The difficulty is modeled after The Lost Citadel of the Scarlet Minotaur ( which is available for free in the Shadowdark Quickstart set). Remember that reaction rolls should be made for many encounters and that not every encounter needs to be a fight!
For parties of less than five, you can bump up the level to two or three, or even just first-level characters with max HP.