Do these units have bonus attacking cities or defeding cities or both?

Both

The bonus applies only when attacking cities. If you look closely, you'll see it says eg. "+1 vs cities". There are two stats pertaining to cities: siege and fortify. Siege gives a bonus attacking cities, and fortify gives a bonus when defending in a city. I don't think any unit has both a siege and a fortify bonus.

No, it's just like other terrain bonuses, only applying on 'urban' tiles, so it counts in both cases.