The term "closed in on" can be replaced by a variety of synonyms, depending on the context. Options may include words like encircled, surrounded, trapped, hemmed in, constrained, limited, constricted, confined, or suffocated. These synonyms all suggest a sense of containment or restriction, indicating a lack of freedom or mobility for the subject in question. Other words that might be used in a similar way include besieged, corralled, fenced in, hemmed, incarcerated, secluded, sequestered, or shackled, each conveying a slightly different nuance or mood depending on the situation. Ultimately, the choice of synonym will depend on what the writer wants to imply or emphasize more strongly in the given text.