Hi Molly
Welcome to 28DL
We have all been in your position and been new to this so I'll help you out
If you watch the Time Team documentry on youtube they show you a tunnel in from the sea, I would recomend a wet suit at least, this tunnel takes you into the main courtyard, its pretty dark so make sure you take a water proof candle or two. Once in the courtyard you have to dodge a few bits and bobs to make your way into the main hospital, the defeat the terrosits and disarm the warheads. Oh actually thats the film The Rock, silly me......
Failing that, work it out for yourself, the rest of us had too