If you use a comma, you are making a semantic decision to include a syntactically nonessential piece of information:
(In the tower,) there is something of interest.
In other words, you're saying:
There is something of interest. Oh, and it happens to be in the tower.
That's fine if that's the meaning you want to convey.
If that isn't what you want to convey, and you want the entire sentence to be syntactically essential, then you leave the comma out:
In the tower there is something of interest.
This version, too, is fine.
However, if you do want to convey everything as essential, I find the inversion to be a somewhat strange way of constructing the sentence.
Unless you are using the inversion for deliberate poetic effect, I would rephrase it:
There is something of interest in the tower.
But if it is being inverted for poetic effect, that's fine too. Notice the difference in mood between these two sentences:
In my heart the evil lurks.
The evil lurks in my heart.
Of course, going back to your sentence, you could leave the inverted order but also add some words to it so the question of a comma is never raised:
(It is) in the tower (that) there is something of interest.
In short, the comma is optional. But its use (or lack of use) will determine the specific meaning of the sentence.
An argument could be made for the use of the semicolon in your sentence. But it would be highly unusual.
With normal grammar, a semicolon separates independent clauses that could stand on their own as separate sentences. Here, that is not the case.
However, sentence fragments can be used if writing stylistically. This can mimic the way that Captain Kirk of Star Trek fame was known to speak:
Look! In the tower! There is something of interest.
So, in theory, if sentence fragments are allowed, then so could be the joining of one or more sentence fragments with a semicolon:
In the tower; there is something of interest.
Having said that, I can't recall anybody actually doing so and, pragmatically, this looks so bizarre and "wrong" that it would make far more sense to explicitly use sentence fragments (if you're going to at all) than to use a semicolon.
Not only is it simply "not done" to use a semicolon this way, but the point of sentence fragments is often the idea of dramatic pauses between parts of speech. Semicolons don't produce that same level of dramatic pause.