Absolutely! Your trip to Trier, Mainz and Xanten will take you on a historic journey through some of Germany's oldest cities. Here are some travel tips to assist you:

Best Time to Visit:
The best time to visit these cities would be the late spring and early fall months, typically between May and September, when the weather is generally pleasant and suitable for outdoor exploration.

Route and Accessibility:
Trier, Mainz, and Xanten are well connected by Germany's efficient rail network. You could begin your journey from Frankfurt or Cologne, both cities have direct trains to these destinations.

Altitude and Acclimatization:
These cities are situated at low altitudes, hence you won't need any time to acclimate. Regardless, it is advisable to take it slow on the first day of your travel to adjust to new environment.

Accommodation and Booking:
From luxury hotels to budget guesthouses, all three cities offer a range of accommodations. It's advisable to book well in advance, especially if you're traveling during the peak summer months.

Clothing and Essentials:
As the German weather can be unpredictable, layering your clothing might be the best option. Pack an umbrella or raincoat for unexpected showers. Comfortable walking shoes are a must, given that these city centers are best explored on foot.

Health and Safety:
Carry basic medicines and a first-aid kit. Since these cities are popular tourist destinations, they have good healthcare facilities available. However, you should get a travel insurance policy for emergency situations.

Local Etiquette and Respect:
Locals appreciate when visitors respect their customs and traditions. When visiting Churches or Cathedrals, make sure to wear appropriate clothing that covers shoulders and knees. Trying a few phrases in German can also benefit you in getting to know the locals better.