You can reach Erice by cable car or bus (very twisty road, hairpin curves, not good if you get motion sick or fear of heights).
Once in Erice, the town is hilly. Not a lot of stairs but the roads are steep in places. If you arrive by bus you'll have to walk maybe a quarter of a mile from the bus dropoff place to the heart of the town (with a lovely church enroute).
It's worth it IMO but it's not an "easy" trip. Strenuous? Depends on your level of fitness and ability to walk.