Neither, because a noun rather than a verb is needed.
The structure of the sentence you are asking about seems to be:
The survey and other things have contributed to…
Insert missing verb — hypothesize or guess
…the development of the archaelogical site (or whatever).
In English “contribute to” must be followed by a noun, rather than a verb; and in this sentence that would be followed by a relative pronoun, “that”. So the verbs would become the nouns “hypothesis” (always preferable in any case, as the original form) and “guess”. In a technical publication the honesty of “guess” is unlikely to attract professional approval, so that “hypothesis” would be the better alternative. However I would consider using the simpler and less pretentious word, “idea”:
The survey and other things have contributed to the idea that the development of the archaelogical site…
Having solved that problem you are left with the fact that the surrounding parts of the sentence are almost incomprehensible. The general solution to such an overweight sentence is to divide it into two. One way of doing this would be:
The wall was surveyed and compared with the masonary of the present-day tower (or whatever the first part of your sentence means).
This led to the idea/hypothesis that the wall-belt evolved (Darwin where are you now? Do you mean was extended over time?) as perhaps did the town(?).