I think it's about a girl trying to accept a new lover after being heartbroken. Therefore she's afraid of a new relationship ("A den or a dessert/Perhaps an ink squirt/A cellar, a wishing well, a war/Or a guarantee will do for me"), but at the same time she tries to find a place in her heart for him ("For you I found a cell/On the top floor of a prison/Just enough space for you to fit your feet in/If you go/Do let me know").
I personally see it in another way.
I think it's about a girl trying to accept a new lover after being heartbroken. Therefore she's afraid of a new relationship ("A den or a dessert/Perhaps an ink squirt/A cellar, a wishing well, a war/Or a guarantee will do for me"), but at the same time she tries to find a place in her heart for him ("For you I found a cell/On the top floor of a prison/Just enough space for you to fit your feet in/If you go/Do let me know").