Confusing Algorithm

[RBotha]

I'm facing the following problem:

"""
In a city of towerblocks, Spiderman can 
“cover” all the towers by connecting the 
first tower with a spider-thread to the top 
of a later tower and then to a next tower 
and then to yet another tower until he 
reaches the end of the city. Threads are 
straight lines and cannot intersect towers. 
Your task is to write a program that finds 
the minimal number of threads to cover all 
the towers. The list of towers is given as a 
list of single digits indicating their height.

-Example:
List of towers: 1 5 3 7 2 5 2
Output: 4
"""

I'm not sure how a 'towerblock' could be defined. How square does a shape have to be to qualify as a towerblock? Any help on solving this problem?

Thank you.