It will depend on the type of tour you are looking for. A private tour? No! One run by the city tourist office shouldn't be a problem, but you will have to check when they are offering the tours, which means you'll have to go to the tourist at least one day in advance.

If you want to do the tour of the excavations beneath St. Peter's Basilica, you need to get tickets well in advance--and it's not easy.

What about tours like Romantic road/ Rothenburg. I checked websited such as Viator and Take tours, but the dates they have scheduled clash with mine.

Viator is a tour consolidator and they offer over-priced bus tours.

Check on Trip Advisor for tours being offered in the cities that you are visiting. Some may be from the city Tourist Info, others may be walking tours or bike tours offered by various local companies, or Hop-on Hop-off bus tours. Walking or bike tours will give you the best experiences for the least amount of money.

If you want to go to Rothenburg, just take the train there and go on one of the tours being offered in the city, like the Night Watchmans tour. This will cost you much less than anything that Viator is offering.

The old-fashioned way to make last-minute reservations, beyond going to the tour operator's office, is local telephone. Still works. Up-to-the-minute, use e-mail from your smart phone or from your small computer over your hotel's free WiFi.

If you plan to be in Athens, check out the tour company mentioned in this article:
http://www.breakthroughgreece.gr/?p=1469

Thank you so much guys. I am relieved!!!
was almost panicking with the thought that I may miss out on alot.
Off the topic. Just booked flights from rome to spain. thinking of spending 5 days in spain. any suggestions?
Would it be possible to visit both Barcelona and Madrid? or would one place be ideal?

Hey, first of all a nice and catchy display name Rumtum. Which parts of Europe are you planning to visit?

its my cats name
Booked accomodations in Germany (booked almost 1 week) and
Italy (booked almost 1 week).
Only booked flights to Spain (planning to stay 1 week) from Italy.
Havent booked accomodation in spain, as I am not sure which city to visit. Barcelona or Madrid?
I would like to visit France and Belgium after spain.
Please suggest what would be ideal for Spain.

thanks