In Ubuntu 15.10 Tomcat6 is in the Universe repository which may not be enabled by default. You can enable it with:
sudo add-apt-repository universe
and then try
sudo apt-get update
sudo apt-get install tomcat6
No luck.
root@srv1:~# add-apt-repository universe
'universe' distribution component enabled for all sources.
but
root@srv1:~# apt-get install tomcat6
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package tomcat6 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'tomcat6' has no installation candidate