Playing along, my plan would be climb the walls and sneak into the keep from the top. Climb speed and BA Dash means I could get from the closest area of cover to the top of the wall while still having my action which is probably a Dash to get off the roof and into the keep, or failing that to find cover on the roof so that next turn I could get into the keep from the rooftop.
But since that's not super fun, how about this, time my approach to the keep to avoid the guards looking my way and just get right up against the wall, ideally on the side that is shadowed so that even though the moon is generally providing dim light inside the shadow it would be actual darkness. Wait there hidden in the shadows expecting that even though it is possible to be seen by the patrolling guards if they have darkvision or a torch, it's still fairly unlikely. Note this is definitely an area where player/dm assumptions might clash, since my assumption is the guards would be focused on looking at the fields so by being right up against the wall I would actually be out of the vision cone unless they stuck their heads over the side and looked straight down.
Once the patrol passed I would then climb the wall and find somewhere out of the guards sightlines which shouldn't be too hard given they are looking outside so even without cover, if I'm quiet and quick enough I can get to whatever door leads into the keep itself.