Siwa is genuinely remote. There is no airport and no railway; everything arrives by road across hundreds of kilometres of empty desert.
From Cairo: a daily East Delta Transport bus runs from Turgoman station, taking 10–12 hours. It's cheap and not luxurious. Many travellers prefer to break the journey at Marsa Matruh on the Mediterranean coast, then catch a connection south (4 hours, several daily buses).
From Alexandria: a direct bus runs once daily, 8–10 hours via Marsa Matruh.
A shared minibus from Marsa Matruh is faster than the bus but more expensive. Private cars and drivers can be hired from Cairo agencies for around $200-300 one way; useful if you're combining Siwa with the White Desert.
Foreigners crossing the Libyan border at Salloum sometimes route via Siwa, but the road south of Siwa to Bahariya is a restricted military zone and requires permits arranged days in advance.