1/ The place is worth seeing. 2/ The place is worth seen. 1/ In this example, the word seeing [gerund] is used as a conditional form of the English verb: to see [infinitive]. You are indicating to the listener or reader a sense of... "Wow! You should see this place." You may have seen the place already or read about it. For the listener or reader, however, it is an action that could happen [you're even recommending it] but it has not yet happened. 2/ In this example, the sentence is incorrect. The word seen is the past participle of see. In an awkward sense it reads like: "The place is worth... [already saw it]" For an English reader it just does not work as a sentence.

The place is worth seen is not correct - that's the difference :) Thge first sentence is correct.