The underlined phrase in the given sentence is <u><em>is older than Westminster Abbey</em></u> The noun phrase that is modified (described) by it is <em>the Tower of London. </em>
A noun phrase is a group of words that contains a noun and functions like one. This means that it can function as the subject, object of a verb, object of a preposition, or a complement within a sentence. The main noun in the given phrase is <em>Tower</em>, and this is why we could say that <em>Tower </em>is the noun modified by the underlined phrase.
