By train
Giessen station is served by high-speed ICE trains. For connection information and time schedule see
http://www.bahn.de
By airplane
The closest airport is Frankfurt am Main (FRA), which is a major international airport, served by almost all major airlines. From the airport there are frequent local trains ("S-Bahn") to Frankfurt main train station. From there you can find local trains ("RE" type, 45 min trip, or "RB" type, 60 min trip) or ICE high-speed trains (40 min trip) to Giessen station.
The airport Frankfurt-Hahn is a smaller airport, served by some budget airlines, however located in a distance of about 125 km from Frankfurt (although the name might suggest otherwise). There are busses commuting from and to Frankfurt main train station. For time schedule see
https://www.bohr.de/en/scheduled-services/bus-routes
From hotel to campus
If you stay at one of the recommended budget hotels, there are public busses to the campus:
Bus 13, direction Schlangenzahl
Bus 3, direction Schwarzacker
The bus stop near the hotels (about 10 min walking distance) is called Oswaldsgarten.
https://maps.app.goo.gl/pqnPs73qQT2jX6VS7
Please get off at the stop Ludwig-Uhland Schule, which is in walking distance to the campus.
https://maps.app.goo.gl/vQAXtAZRs3NGGaA28
Link to the bus timetable:
https://www.swg-verkehr.de/fileadmin/user_upload/PDF/04_Verkehr/01_Fahrplaene/Fahrplaene_2024/2023-11-01_Linie_3-13.pdf