from fastapi import APIRouter, HTTPException

towns = [
    "Douala",
    "Yaoundé",
    "Garoua",
    "Bamenda",
    "Bafoussam",
    "Nkongsamba",
    "Maroua",
    "Ngaoundéré",
    "Kumba",
    "Edéa"
]

router = APIRouter()

@router.post("/open")
async def get_towns():
    """Returns list of towns in Cameroun"""
    if request.method != "POST":
        raise HTTPException(status_code=405, detail="Method Not Allowed")

    return {
        "method": "POST",
        "_verb": "post",
        "towns": towns
    }
```

This code defines a list of towns in Cameroun, then creates a FastAPI router with a single POST endpoint at `/open`. When this endpoint is called with a POST request, it returns the list of towns along with the request method metadata.

If the request is made with any HTTP method other than POST, it will raise a 405 Method Not Allowed error.

The response format adheres strictly to the provided examples, including the `method`, `_verb`, and nested data structure under the `towns` key.