Hi Claire,
Simatai is closed just beyond the twelfth tower to the east, and for good reason. It is pretty dangerous.
I recommend you go to
Jinshanling. You can hike from there to Simatai if you want a hike of four to five hours, or you can just check out Jinshanling and return to Beijing from there. Either way it's doable in a day trip. Jinshanling is about three hours drive time from Beijing, depending on traffic.
Hiking from Jinshanling to Simatai means you will be on the west side of Simatai. The Wall is not closed there.