it was quite clearly heavily guarded , and although the two of them managed to clamber undetected up scaffolding which had been erected on the side wall of the cathedral which adjoined the palace , when they looked over from their vantage-point they could see that even though they could - and did - leap across on to the palace roof , access to the courtyard , even from there , was barred by a high grille whose spiked top curved outwards and downwards .