First, I'd like to say good job getting the site up and available. I know it can be a lot of hard work so my suggestions are meant to help you improve the experience. You've received several good suggestions and I'll add/confirm my observations.
1) Alt text is nice, but you can easily add it at any time. Definitely not a deal killer right now and you have other issues that need some attention.
2) Why so dark? It's not mysterious, it's just dark and hard to read/see. Even for those of us that can still see 20/20 it's a struggle.
3) Heidelberg - I love the city, but after seeing your first shot, you showed it to me again, and then again. Or was there a difference? OK, a little sarcasm there, but if you're going to highlight the city, then show the city, the fort, the powder magazine, etc. Make it interesting and not repetitive.
4) L TOC/navigation buttons - I felt that I was looking at the History Channel's web site and I was about to explore Roman Ruins. Sorry, that was just my initial, and repeated reaction. It's all personal so leave it if you like it.
5) Low contrast text - the "welcome" text is almost invisible. there's subtle, and then there's asleep. If you want to convey a feeling without overstating things, you'll need to be careful, but no matter what you need to pull it up a notch.
6) Impressum - Huh??? English is the most common universal language (sorry to the French). You not only incompletely mix English and German on the page, but you're going to lose a huge segment of your desired audience. Add a full English section and keep the German too.
7) I like your landing page, but I don't like to have to click again to see what i was already promised. The tease becomes a nuisance.