Castello Scaligero in the commune of Sirmione sits on a thin peninsula jutting out into the southern end of Lake Garda. Essentially, the ramparts of the castle function as a small fortified harbor for what was the Scaligera fleet. The castle has a de facto moat (the surrounding lake) and even a drawbridge through which visitors enter. There are 146 steps which take you up to ramparts of the walls which look down upon the little harbor. There is a small walkway around which bowmen would be stationed to ward off any intruders from the lake in addition to defensive towers.
Castello Scaligero is perhaps the best preserved castle from medieval times in Italy, constructed in the middle of the 13th century on top of the remains of an ancient Roman fortress. Taking over a century to build, two courtyards and an other fortifications were also added. In 1405 the Republic of Venice took control and strengthened the castle even further.
If you decide to visit, consider that in high season Lake Garda can get very busy. Still, there is so much to do in the surrounding area, even water parks and campgrounds if you're so inclined:
Sirmione Historic Center - After visiting the Castello, take a stroll around the old town of Sirmione. There are shops, restaurants, gelaterias, pizzerias and beautiful architecture. Be sure to take in a passaggiata (stroll) at dusk to Piazza Callas and its pier that juts out into the lake.
Archaeological site of Grotte di Catullo - Ruins of a sizable Roman villa on a peninsula with an olive grove & a nearby archeological museum. Can get crowded in high season but worth a visit.
Camping Village San Francisco
Mantua & Peschiera del Garda - Mantua is a wonder town surrounded by manmade lakes which create a huge moat. Peschiera del Garda is a "water" town--surrounded on all sides by the River Mincio which connects to the lake.
Lake Garda Cruises
GVI Travel Tip: Best visited in the off season.