Not really a fortress
Most guidebooks call Sacsayhuaman a fortress, but Cusqueñian archeologist René Pilco Vargas explains that it was actually a temple.
At Sacsayhuaman, archeologists found obvious offerings from all regions of the Inca’s territory throughout South America. This points to the structure being a temple, not a military fortress.
When you visit (which you absolutely must), you will see why the confusion. The walls are massive, made with giant stones weighing up to 125 tons.
There is a free section that you can walk through from Calle Siete Borreguitos, but to get close to the main walls you have to buy the Boleto Turístico de Cusco or take the guided City Tour that every agency in Cusco offers.
There are two entrances. On the east side, Rumipunku is where you’ll enter if you walk up from Calle Siete Borreguitos, or if you go with a tour. On the west side, Llaullipata is the easiest place to get if you take a taxi from the Plaza de Armas.