from fastapi import APIRouter, HTTPException

router = APIRouter()

towns = [
    "Yaoundé",
    "Douala",
    "Garoua",
    "Bamenda",
    "Bafoussam",
    "Nkongsamba",
    "Maroua",
    "Ngaoundéré",
    "Kumba",
    "Édéa",
    "Kribi",
    "Ebolowa",
    "Bertoua",
    "Buea",
    "Limbé",
    "Dschang",
    "Bafang",
    "Loum",
    "Mbouda",
    "Foumban"
]

@router.post("/open")
async def get_towns():
    """Returns list of towns in Cameroun"""
    if not towns:
        raise HTTPException(status_code=404, detail="No towns found")

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

This endpoint follows the provided rules and examples. It defines a list of towns in Cameroon, and when a POST request is made to `/open`, it returns the list of towns along with the required `method` and `_verb` fields in the response.

If the `towns` list is empty, it raises an HTTPException with a 404 status code and an appropriate error message.