I have a town, the capital of an empire. It covers 560 Hectares.
What kind of natural barrier conforms to the following requirements:
- Is both heavily defensible and does not hinder trade all too much.
- The technology is of 1200 AD Europe.
- A large (Rhine sized) River either runs through this town or starts in/adjacent to the town
- Only has 2 or 3 passages into it from the land (used by everyone daily to get in and out of the town, they should be wide/tall/open enough to have main roads on them and can be natural or artificial), however these passages are not very large (1-2 miles long)
- Last but most important, the barrier with only 2 or 3 passages through must be strategically implausible to circumvent. This town should not be able to fall by way of force unless you swarm the passages with thousands of men.
I wish to know what type of natural barrier I could use to make it realistic both geologically (it must be able to be explained how it naturally formed) and strategically (it must not prevent trade or be too easy to overcome with an army).