IIRC, it's about 50 metres from where the bus drops passengers off - and flat - to the entrance turnstiles. From there it is all walking on relatively gently sloping ramp up into the citadel area itself. Within the citadel walking areas are mostly flat but very uneven and some steps/stairs are taller than usually found in homes or offices. But once inside it is a relatively easy walk - assuming one can walk without much trouble over uneven grass and rocky/stone surfaces - note that there are no handrails or guidelines. Walking sticks or canes WITH RUBBER TIPS are allowed but only when necessary due to age or infirmity - otherwise walking sticks/hiking poles are prohibited, as are umbrellas. You don't qualify what a "little knee problem" means, but for most people the problem walking around is not actually climbing the steps or walking across uneven stone paving - it is simply being in poor physical shape in general let alone trying to walk at altitude.

Other notes: 1) The only public bathroom is just outside the entrance turnstiles - there are NO bathroom facilities once inside so be sure to go before entering! 2) Bring a poncho or packable rain jacket - umbrellas are not permitted.

Best time to enter the first day you are there is in the afternoon, most of the crowds will have left (it is most crowded from 8am-2pm) so you have the best photo opportunities with less crowds and the golden hour of sunlight.