What was Holmes's original plan?

First of all, remember that it's Sherlock Holmes, so we're supposed to accept on faith that everything Sherlock imagines will go exactly according to plan. Remember the end? He jumps off a cliff just because he imagines himself losing the fight with Moriarty. So in that context, it's reasonable to guess that maybe Holmes knew exactly what would happen and was just messing with Watson, as he often does.

Another explanation may be that he hoped Watson would be able to see him from on top of the tower. Remember, a big part of Holmes' plan revolved around his interpretation of the pictures they found, one of which was the view staring out through the torture chamber ceiling at the lighthouse. The ceiling of the chamber was glass, meaning anyone from the lighthouse would be able to see in as well. He probably hoped that the tower provided the best means for Watson to see where Holmes was without being in danger - after all, the only other way would be to go into Moriarty's lair himself. From the tower, then, there are several things Watson could have done - for instance, he could have sniped Moriarty and friends.

It's also a common trend in Hollywood for movies to make sheer luck look like part of some grand plan - "The Dark Knight" is the best example I can think of. So that's probably actually what happened.