Q:

Why is there an indefinite article in this sentence?

Why is this not the sea-lion pool with a definite article instead of a sea-lion pool with an indefinite article? 

And the decision to build the zoo around a sea-lion pool was the crowning touch.

It is a particular pool, and it has been mentioned in the text before, so isn’t that the wrong kind of article to use here?

A:

Caro invites you figuratively into the planning—you are to imagine Robert Moses saying at that time "We're going to build this around a sea-lion pool". When the decision was reached the pool didn't exist.

A:

Both 'the' and 'a' are fine, but express different things. 
When using 'a pool' it implies that at the time of the decision there was no specific sea-lion pool to talk about. It hints they did not decide for this particular pool, but any pool. Maybe this particular pool was designed later by the architect.
When using 'the pool' it would imply a decision to build this particular design for the pool.

