First built in 474 BC, Castello Aragonese is an incredible attraction, a large, imposing castle built on a rock off of the main island of Ischia. In the 15th century, a stone bridge was built to connect the two and is still in use today. Inside, there are churches, gardens, terraces, and even accommodations for visitors.
Less an hour walk from the port, you can take the stairs (surprisingly, not too taxing) or a lift to the castle, which is renovated in some spots, but which lies otherwise in the type of ruin all kinds of tourists come to appreciate. Despite it being the most popular attraction in Ischia, and a 10 euro (about $13 US) entrance fee, you could spend a good half day idly exploring the castle and reveling in its ancient grandeur.
If you've got a little time to kill at work and you're sick of playing Minesweeper, check out our Ischia adventures for lots more to do on the island.