There are free to access castle ruins and buildings of a similar age and much older all over the UK, they're hardly in short supply. Maybe you didn't intend it but this comes across as overly pessimistic. It's hardly as if every bit of history in the country has had a fence put round it to charge an entry fee.

Not trying to say what you're describing doesn't exist, but the only place I can think of that I've been to that charges to see some 3ft high bits of wall would be Old Sarum, but that also has great views.