Two churches in Cologne. Alt St Alban has no online ticket listed; Great St Martin Church has 1 ticket or tour to book. The table and the two notes below put them side by side; the full pages have what to see, the ticket and the timing for each.
There is nothing to buy: the church is a roofless ruin kept as a war memorial, open to the sky and enclosed by a low railing, and it can be looked into at any hour from the courtyard beside the Guerzenich concert hall in the middle of Cologne. No ticket, no opening times, no guide. Walking tours of the old town pass it and usually stop for a minute; those cost a moderate amount and cover a dozen sites.
The church is free to enter. What costs a small sum is the guided descent into the excavations beneath it, which show Roman warehouses, a swimming pool and the remains of the island the church was built on; those tours run at fixed times, in German and sometimes English, and are the most interesting thing here. Walking tours of the Romanesque churches of Cologne include Great St Martin and cost several times that;