Climbing over rocks and such while invisible might be a bit difficult... I'm beginning to think that we should go thru the main gates, but am open to trying the south wall. Either way, we don't have terribly long while invisible once we reach the keep -- only about a minute and a half.
And if we open any doors, anyone there will obviously know something is there, but Virashil can use dancing lights as a distraction/diversion -- make anything there think the lights/glowing mass is the intruder and not us.
Dancing lights also won't break the invisibility.
Dancing Lights
Evocation [Light]
Level: Brd 0, Sor/Wiz 0
Components: V, S
Casting Time: 1 standard action
Range: Medium (100 ft. + 10 ft./level)
Effect: Up to four lights, all within a 10-ft.-radius area
Duration: 1 minute (D)
Saving Throw: None
Spell Resistance: No
Depending on the version selected, you create up to four lights that resemble lanterns or torches (and cast that amount of light), or up to four glowing spheres of light (which look like will-o’-wisps), or one faintly glowing, vaguely humanoid shape. The dancing lights must stay within a 10-foot-radius area in relation to each other but otherwise move as you desire (no concentration required): forward or back, up or down, straight or turning corners, or the like. The lights can move up to 100 feet per round. A light winks out if the distance between you and it exceeds the spell’s range.