I really liked the office and engineering teams I talked to during my on-site interview.
The interview questions asked were at about right difficulty and some of them even had a connection with real world coding which is rare since the inception of Cracking the Coding Interview book.
Unfortunately, I can't say anything good about their Technical Recruiting. A guy I worked with was the most shady person I met. Also, I had to come for the follow up on-site interviews multiple times.
The whole process took more than 3 weeks and I was never contacted by anyone at Squarespace again. I can live with not getting the job, but for them to not have the courtesy to let me know was just plain unprofessional. My advice is to look elsewhere for a job where you will be treated with more respect.