In my backyard...

[kristinhaze.livejournal.com]

So, I'm still relatively new to Davis Square and I'm a busy bee and hardly have the time to stop in the park area behind my street and find out what's going on there but I pass by it sometimes and am very curious...
the park area that is across from Powderhouse Circle (I believe its "back" is on Liberty Ave. and "side" is on Broadway)contains a tower-like structure...can someone tell me what this was/is used for?

Thank you!
Kristin