Ambleside Roman Fort is not spectacular in the ruinous-castle sense. What survives is mainly low stonework and earthwork lines in a field near Waterhead, but the setting does the work: lake, valley, and fells all explain why the Romans put a fort here.
It is worth visiting on the walk between Ambleside and Waterhead, especially if you like places where the map suddenly makes history obvious. Combine it with the lakeshore and Waterhead Pier rather than treating it as a stand-alone expedition.