This mountain use of a void sim may be a great way to create a new community project in addition to the planned 4th sim. The economics of a void sim as a rural landscape are intriguing. and it would help balance the landscape with open and urban spaces. So, as a wild estimate: 415 usd purchase, 6 small well placed view lots at 80-90 usd, and a tier of 112-16 usd. at first blush this looks viable, but I am not knowledgeable in sim finances, just speculation.
Looking at the terrain on both the east and west sides of NFS, the west side slopes deeply into the valley and would require a VERY massive headwall. Such headwalls are almost always the result of glaciation and the steep walls tend to face north/north east. That side of the terrain tends to be in the shade during the annual sun cycle, hence the glaceirs grow there, cutting away the rock more rapidly as climate warms exposing steep dramatic mountain walls (with notable exceptions as the jungfrau valley and Yosemite which have headwalls on both north and south and have very complex geological origins) by having mountains, snow and ice above the town wall we also build a more plausible account for the nfs/am river.
Another plausible location for a small mountain range might be to the east of the city walls with a small mountain range rising close to the south end of NFS and running to the north east sloping possibly in a series of cascading summits of snow and ice and granite down to meeting alpine meadow lands at a future sim to the north of the void and to the east of Alpine Meadow. the mountain sim could then drop away rather steeply to the sea forming a headwall to be part of a potential Fjord lands sim further east .
we can create a small valley between the city walls and the mountains where a few pesents might have had a dairy. While castles normally occupy hig grounds, they were usually not built in the real summits (look at how late the summits of Mt. Blanc and the Matterhorn were reached. NFS should guard a mountain trading route, and this would give us that mountain pass.
futher, only two property owners would be impacted by this placement.
Just speculation, but fun i think. and if we do not consider such ideas now, they may be inadvertently eliminated by the planning process itself