The highest is an example of a superlative adjective. To find out what superlative adjectives are and how to use them, follow the link.
Notice that the past tense
can sometimes be formed with did
instead of using the past tense form of the verb. So you can say "The tower opened
on April 27th 1904." or "The tower did open
on April 27th 1904.
Here used to means 'in the past'. Follow the link to find out what else it can mean.
A garden that has been let go has been allowed to deteriorate and become overgrown.
Example: The garden was let go after his wife died.
If you take something upon yourself, you decide to do it, often without asking anyone if you should.
Example: He took it upon himself to collect the mail for the office every morning.