Cologne - While the Bundesliga's many stars continue to build up their fitness levels ahead of the new season, clubs up and down the country are working tirelessly behind the scenes to strengthen their playing staff in time for the big kick-off in mid August.

bundesliga.com is taking a closer look at each of the 18 top-flight clubs' summer transfer dealings so far. Next up in the series: Bayer 04 Leverkusen.

How they fared in 2014/15...

Bayer 04 Leverkusen did not drop any lower than sixth last season. After Matchday 5, however, they never rose any higher than third with Roger Schmidt's men leaving a marker in fourth place, and qualifying for the play-off round of the UEFA Champions League. In the words of their sporting director Rudi Völler, that is about as high as they can aim given the strength of the competition, so it was therefore a satisfying season for die Werkself, and one they will be happy enough to emulate in 2015/16.

Ins...

Christoph Kramer (r.) (Borussia Mönchengladbach)

  • Part of Germany's 2014 FIFA World Cup-winning side
  • On the Leverkusen books since 2010, but yet to make his first-team debut for the club

What he offers: Kramer is back from a two-year loan spell at Gladbach where he developed into a key player for club and country. Does everything in midfield, defensively and attacking. Excellent in tackle with great vision.

Admir Mehmedi (SC Freiburg)

  • Capped 33 times by Switzerland
  • Scored 16 goals in 60 Bundesliga appearances.

What he offers: Physically strong striker with a sharp eye for goal, capable of scoring with either foot and his head. Secondary striker who can complement Stefan Kieẞling

Outs…

Simon Rolfes (r.) (Retired)

  • Made 377 appearances for Leverkusen, 288 in the Bundesliga
  • Bowed out as Leverkusen captain

 Gonzalo Castro (Borussia Dortmund)

  • Veteran of 286 Bundesliga appearances
  • Provided nine assists and two goals last season

Stefan Reinartz (Eintracht Frankfurt)

  • Comfortable in defence or in midfield
  • Played 145 times for Leverkusen

To-do list…

"We're still going to do something in one or two positions – certainly in midfield, and perhaps also up front," said sporting director Rudi Völler (l.) to the Kölner Stadtanzeiger. "But we've completed our most important construction at the back." Kramer has come in for Rolfes and Jonathan Tah for Reinartz, but Castro has yet to be sufficiently replaced and that could be the area where Leverkusen need to strengthen. A lot may hinge on whether or not they qualify for the group stage of the UEFA Champions League, so the final few days of August could see a flurry of activity in Völler's office as the finishing touches are added to what already looks like an exciting team with a reduced average age compared to last season.

Editor's note: Not all summer transfers will be discussed here. The players featured in this article made at least ten league appearances for their respective clubs last season, chipped in with numerous goals and assists or made other significant contributions to their team. For an overview of the 2015/16 summer transfers, click here.