I spent 3 days in Lisbon during shoulder season, and there are 4 things everyone should know before they book a trip there

• Lisbon, Portugal, was named the most popular travel destination for millennials, but there are four things you should know before you go.
• Be prepared: Lisbon is bigger than it looks and a bit colder during shoulder seasons than I expected.
• Lisbon is a great place to visit if you want to see a foreign city but feel uncomfortable with language barriers.
• The recommended day trip to the "fairy tale castle" in Sintra isn't all that magical.
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Millennials are loving Lisbon, and I don't blame them.

It was named the most popular travel destination for millennials in 2019 by travel-planning site müvTravel. I visited the hotspot in September to see what all the hype was about.The city certainly gives millennials what they want when traveling: an affordable experience that has great nightlife and restaurants, and is - perhaps above all - Instagrammable.

While I would recommend Lisbon to anyone thinking about booking a trip, there are four things they should know before buying that plane ticket.

1. The city is bigger than it looks on a map.

I was warned to bring a pair of walking shoes for Lisbon's extreme hills. "It's like San Francisco," I was told multiple times.

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I didn't realize how big the city actually was. It looked pretty compact on the map, but some of the city's neighborhoods were more spread out than I anticipated. In retrospect, I should have known better, considering that Lisbon is Portugal's largest city.

That Lisbon runs far and wide is compounded by the fact that there are, indeed, many hills. There are also many winding alleyways. Walking up or down a steep incline or a set of stairs and getting lost in the streets can make walking one mile feel more like two.I didn't even make it to all the neighborhoods I wanted to visit - if I had done my due diligence and realized how big Lisbon was, I would have planned my itinerary a little differently.

I highly recommend staying in the center of the city. This is something I always try to do, but I booked and planned my trip about six weeks in advance. By the time I booked an Airbnb, the ones located in the center within my budget were unavailable. (But be warned: If you do stay in the center, you'll likely have to drag your suitcase up some steep stairs. It's all about the trade-off.)

I ended up staying about a 25- to 30-minute walk from the center.

Luckily, Lisbon has a Lisboa card, which grants you access to all of Lisbon's public transportation (metro, bus, train, and trams, including travel for day trips to nearby Sintra and Cascais) for a daily fee.

Read more: Lisbon is the hottest travel destination for millennials in 2019. We asked 20-somethings for their best travel tips - from fairy-tale castles to buzzing nightlife, here's what they said.

2. Shoulder season has its perks - but also its drawbacks.

June through August is prime season for visiting Lisbon (as with most European cities). US News & World Report recommends going during shoulder season - March through May or September through October - for cheaper rates, fewer tourists, and warm weather.I have to agree on all those points except for the latter. Our Airbnb was around €66 per night ($72) - split with my friend over the course of three nights, I ended up paying just under €100 ($110). And while we were far from the only tourists there, it didn't feel mobbed.

But the weather was finicky. It was the time of year when there's a 15-degree difference during the day and you just don't know how to dress to stay comfortable. The temperature was in the early 60s (°F) in the morning and late 70s (°F) in the afternoon.

What really got me was the wind. It felt nice in the sun - I broke a sweat exploring in the afternoon - but in the shade, the wind was so strong that it immediately felt chilly. It was even stronger near the coast: A windy, sunny 75°F day - weather I bounce around in jacket-free at home in New York City - had me shivering.

You'll definitely need a tank top - and a jacket.

3. It's an easy place to travel if English is your only language.

Lisbon is very English-friendly.

I started this trip out in Madrid, Spain. While there were some signs and menus in English, and many Spaniards seemed to know at least a little bit of English, it was a sharp contrast from Lisbon.

For example, the metro in Madrid didn't have an English option, but the one in Lisbon did. That makes a big difference when getting around in a foreign country. Many of the signs around Lisbon were also in English.

Read more: I spent 3 days in Lisbon, Portugal - and I totally get why it's the most popular travel destination for millennials in 2019

I also didn't meet a single resident in Lisbon who didn't speak English, whether they were a waiter or an Uber driver. I went on a sunset cruise of the city, and the deckhand was so fluent in English he sounded American.

All the restaurants I went to in Lisbon offered menus with an English version (as well as a few other languages). My travel friend, who lived in Madrid for three years, also commented on the difference between the two cities.This is not to say that Lisbon or Madrid is a better travel destination than the other. My point is just that if you want to explore a foreign destination or experience a new culture but are uncomfortable with language barriers, Lisbon is a good place to go.

4. The recommended day trip isn't as great as you think.

Browse Trip Advisor, Yelp, any forum thread, or my previous article on Lisbon travel tips, and you'll come across recommendations to take a day trip to Sintra, home of the "fairytale castle."

Let me tell you something: The castle in question, Pena Palace, certainly looks like it's from the pages of a storybook - but it is not all it's cracked up to be.

My friend and I arrived in Sintra around 10 a.m. We thought it would be wise to get there early, before the crowds showed up. Unfortunately, the crowds had the same idea.

As you can see in the image above, the line to get into Pena Palace was extensive. We waited over an hour to get inside the palace and tour it.

My friend and I were both annoyed with the wait, but she brought up a good point - it may have been better received if there were fine print on the ticket or website saying that a wait is likely. It would have allowed tourists to better plan their day accordingly.

It was a cool experience, but I wouldn't do it again. And if you're only in the area for a day or so, I'd recommend skipping it altogether to get the best use out of your time.