Credit to Indian Knight/Giant Killer for the content of this guide.
Indian Knight from Indian Server is the same person as Giant Killer in uk forum
Introduction to building and functions
The horse drinking trough of the Romans decreases the training time of cavalry and the upkeep of these troops as well. Also it can be built in Roman wonder of the world villages.
Prerequisites:
Stable level 20, Rally Point level 10, Tribe: Romans, Travian Version 3.5 or higher
The horse drinking trough in detail:
1. Can only be built by romans.
2. Accelerates the production time of cavalry units by 1% per level.
3. The horse drinking trough lowers the crop consumption of all roman cavalry units in that village.
4. Equites Legati consume one crop less at level 10.
5. Equites Imperatoris consume one crop less at level 15.
6. Equites Caesaris consume one crop less at level 20.
7. The maximal level is 20.
Before the HDT the EI's had an offensive value per crop upkeep of 40 while the EC was 45...so, according to this long term perspective, the EC's was the best offensive cavalry of the roman empire before HDT
After the HDT gets put into place the offensive value per crop upkeep of both troop types are 60 - so completely equal. This shows that you can save the last 5 expensive upgrades of the HDT if you look only at the long term offensive value.
Of course, if you build it to level 20 you get 5 % faster training time. And the EC is also slightly better at defending - although this troop type shouldn't be used for that.
What suggests using the using the EI instead of EC's is their travel speed, their ability to carry more resources and their faster training time.
EC's are better without the HDT, EI's are better with the HDT. If you choose to use EI's in your hammer, consider building the HDT to level 20 anyway. It improves your ability to build ur hammer and greatly improves your CP prod...!!