Across the Vltava River, overlooking the Lesser Town (Malá_Strana) is the Prague Castle (Pražský hrad), the largest medieval castle in Europe and the seat of power of the past kings of Bohemia, Roman Emperors, the current official residence and office of the President of Czech Republic.
It might look far to reach but going to the castle is actually quite easy.
There are five ways to reach the castle complex as published in the castle official website (https://www.hrad.cz/en/). The provided options mainly involves taking the tram, however everywhere in Prague feels like a mere walking distance away, so I just walked my way from the Old Town via Charles Bridge, explored the Lesser Town and headed the Prague castle by the Old Castle steps.
Prague Castle is huge, with an estimated size of a seven football fields.
A complex of churches, gardens, courtyards and alleyways, to see the castle, many visitors either participate in one of the walking tours while some go for the self-guided approach and simply purchase the combination ticket to enter the historical buildings.
Actually, Prague Castle can be explore for free but it will be only limited to the courtyards. If you happen to be at the complex later in the afternoon, you might chance upon that the Golden Lane (Zlatá ulička) is actually free to enter at around that time.